Missed period, not pregnant.

I'm sure this question has been asked probably many a times, but I am a 21 year old female, I am 5'5 150 pounds...I used to be 130 but I skipped my period for about 5 months now, and I gained A LOT of weight in that time. I am not pregnant obviously,...

but I really don't know what else to do, My doctors don't really say much...Just that your not pregnant don't worry! I really want to get back to my normal weight, but it's hard because I don't get out much, and I don't have a job. (For medical reasons) I was just curious, if anyone knew WHY I am skipping my period. I noticed that when I usually skip my period, it's usually the following period I have after I've had sex. I'll have sex, then miss my next period almost like my boyfriends penis is pushing a period stop button inside me haha. I really don't know why I miss my period, I eat about the same as I usually do, but I am a night owl, could that be the problem? Staying up too late? I'm still young so playing video games at night is something I do like all the time lol. Help!

well you could be oregnant
there is no 100% guarentee that your safe when you have sex
weather you use protection or not, your still not 100% guarenteed to not get pregnant
so it is a possibility
unless the doctors have taken a blood test and determined that your not pregnant
if they havent, ask them to
and take a pregnancy test as well to make sure
other than that its probably a combination of( I assume your on birth control) and not exerscising
getting regular exerscise, at least half an hour a day is the daily recomened amount
weather you do it while walking to a store, or on a treadmill/exerscise bike while watching tv,m as long as your getting active you should start noticing a change
it would be a good idea to start exerscising while your playing video games too

the same thing happend to me and I made the obgyn give me a ultrasound .I had a blockedge and had had it for 4 months they gave me some pills and I started in a couple of days.GET A ULTRASOUND!!!