Ya'all, help a sister out

Ellen
Hey everyone, I'm posting a question. Let me explain my situation... I'm a 21 year old college student who is sexually active. My main form of birth control is the pill. I almost never use condoms, and deal with STDs by remaining  monotonous with whatever partner and by being upfront about STD testing. I take my pill at the same time every night and am very good about not forgetting. However, this summer I knew I wouldn't be sexually active so I was really infrequent with my birth control for the first time in the three years I've been on the pill.  When school started again in August I went back on my strict regimen. I do the Sunday start, but when I started by period in August it was on a Sunday, so I took a pill that evening. My period was normal the following month. 
However, the month after that I missed one pill in week 2, and had unprotected sex five days later. By the time my placebo week rolled around I completely missed my period. I took a pregnancy test and it was negative, so I resumed my birth control like normal. The following month I was sexually active once (no condom), but got my period as usual. (Usually on Wed/Thurs and about 3 days long) The following month i was not sexually active, but completely missed my period again. On the Sunday I was supposed to start a new pill pack I took a pregnancy test in the morning (the first pee of the day) and the test immediately came up as negative. Now this month, I have not had sex, but am in the end of my placebo week and have not had my period. I know it's highly unlikely that I'm pregnant, but I am worried.
It might help to add that I live a high-stress life, so that may impact things. 
What do ya'll think??? Does anyone have a similar story? I'm going to grad school in the fall and am not ready to deal with an unplanned pregnancy