Worried... Late period.

So I've always been someone to have a heavy period. It's never been the most consistant thing ever but I've never missed a month before. It's been late by a few days here and there but never missed one. My last period I had was Sept 24 and it was abnormally short and very light. I just figured it was from moving back to school, being at school, having a new sleeping schedule, etc. Well, October 24th came and went and no period. I started freaking out mildly and it's been 12 days now and still haven't had it. I've taken 3 pregnancy tests (two HPTs and one from planned parenthood) and all were negative (they were all done one three different days). I am sexually active but me and my boyfriend always always always use condoms and pull out every time. Yesterday I started feeling like I had cramps for like 2 minutes then it would go away and come back and keep repeating and repeating. I put a tampon in yesterday and after about 2 hours I took it out and there was verrrrrrrryyy very minimal something on it (red/brown color) so I got excited and thought maybe my period finally started! But then this morning there was nothing and now tonight there was the same thing as before but even a little less. I've been pretty stressed this pasted couple of days because I would not be ready for a baby due to being a full time student. Do I have something to worry about? I keep thinking "what if the tests are wrong and not detecting the hormone!?" and stuff like that. Has this happened to anyone before? I desperately need advice before I make myself go crazy!!!

Thanks in advanced!