Reasons for being late for my period?

Heres the deal, I have a boyfriend and we have sex a lot but we always use protection. I am three days late on my period could any of this be the reason :

1. I smoke a lot of pot. (almost daily)
2. I weigh 245 pounds
3. I am a little stressed about school (I'm a freshman in college)
4. I thought I was about to start my period but we had sex anyway, then it didn't come. We didn't do it for one day and I got the feeling I was going to start again (cramps, being emotional etc) and then wehad sex again and the feeling went away.

Can having sex repeatedly (2-3 times a day) whenever you feel like your about to start your period make it late? You know how the day before you start you can feel cramps coming and your mood changes and all that. If you have sex a lot the day before and then the next day and so on prolong your period starting?

Yeah it can - also stress and body changes can have an impact.