What Causes Delay In Periods?

I don't know what to do about this but...
I got my period in the beginning of February and got another one a week or so later. I figured my hormonal system was all messed up so I stopped taking the pill. I haven't had any sex since I stopped but STILL haven't gotten my period. Also, I've taken 2 pregnancy tests and they both said that I'm not pregnant. I have been stressed with school and for a trip I'm going on. What should I do?