My boyfriends mom thinks I am not right for her son

I have been dating my boyfriend for about 3-4 months now(sorry I am not a counter) anyways I thought his mom liked me up until last night... Kevin had just gotten out of a serious 4 year relationship in which he was engaged to the girl and she was jobless and living with him and his parents... Needless to say things went downhill between them not because of me but because she cheated on him and a few other issues... Now I will admit that I cheated on my last boyfriend with Kevin but rest assured I learned my lesson and that was the first and last time that will ever happen... Now even though I have my own apartment and recently got a new job doing Jewelery sales which is what I am going to school for (Gemology)I am over there pretty much every night... Kevin and I have a mutual friend Denzil and a couple of times Denzil has come to Kevin's house to pick me up to either run erronds(I don't have a car mind you) or look at apartments in Loveland since I will have to move out there at the end of October for my new job and Denzil was looking to move out of his parents house so we thought it would be cheaper to get a two bedroom together and yes I invited Kevin to move in as well... Now since Denzil picked me up at Kevins house his mother obviously saw him and thought I was cheating on her son with Denzil mind you we have always invited Kevin along to view apartments or hang out and he has declined so its not like I am going behind his back or anything like that and Kevin trusts me and Denzil and knows nothing would ever happen but as for mommy dearest apparently she thought differently as she expressed to me she also told me that she didn't think Kevin and I should have gotten into a relationship so soon after breaking up with his ex which is another dilemma...his ex makes me want to commit murder on a frequent basis she is constantly calling Kevin and asking him to do favors for her now at first I just blew it off then Kevin started to lie to me about them talking and hanging out so I confronted him and he said that he thought that if I knew he was talking to her that I would get all bent out of shape and didn't want to upset me or hurt my feelings which I understood however I was still upset that he lied to me and didn't think I was mature enough to handle him being friends with his ex since then everything had been going great up until last night since I recently quit my job because I will be starting my new job tomorrow Kevin had to still go to work now be considerate since I am practically living with him I cleaned his room which I have done before and went out to get food to make him dinner(also something I have done a couple times prior) while I was out I picked him up chocolates, a 4wd trail book(something he's really into),razor blades for his razor(he always complains that there too expensive), and a pack of chapstick(he is obsessed with the stuff and always loses it)...sound like a pretty nice girlfriend now don't I? So after making him dinner and surprising him with the gifts he actually tells me well this is going to awkward because I told my ex that I would go to her house and watch the ring two with her!!! I nearly flipped a lid I have never been so angry and hurt in my life I couldn't believe after all the nice things that I had just done for him and for no particular reason or anything I wasn't intending to get something out of it just being nice he had the nerve to lay that on me... I told him to just go and stormed out I needed to go for a walk to cool down considering I was practically sobbing and I hate people to see me cry so I left called my mom who cooled me down and rationalized with me convincing me to go back and talk it out with him so I did needless to say he told his ex no and I told him I didn't care if they talked on the phone occassionally or went out for coffee or lunch or dinner every once in a while but the fact that he was going to go over to her house late at night and watch a movie with her was just pushing it too far... I told him isn't that what her new boyfriend is for? Now after all that I just would like to know why I am getting the shit end of the stick from mom after all I do have my own place am not jobless and try to help out around the house and even buy my own groceries as much as possible yet she still dislikes me even though I haven't been doing anything wrong yet Kevin seems to screw up and he's perfectly innocent??? Also what am I suppossed to do about the ex-girlfriend I don't want to be a bitchy and controlling girlfriend and say you can't talk or see her because I have had controlling ex boyfriends in the past and wouldn't want someone to do that to me again much last do that to someone else... I just feel that she is constantly getting in between us and don't know what to do???