Pregnancy or just a late period?

 My period was supposed to come late April (I believe the 25th or something) but it hasn't come yet. My boyfriend and I haven't had sex, and we're careful to make sure my vagina is far away from his penis if we're both naked. We give each other oral, and I use a condom when I perform on him. I'm paranoid that some sperm somehow got on my thigh and wriggled its way into my vagina. My period has been late before, I think 2 weeks is the max, but I'm still really worried. I haven't shown any symptoms of pregnancy except for a missed period. I plan on going on birth control when I become 18 (this month). I want to wait and see if it pops up later, but I don't want to miss the window of an early healthy abortion, but I'm so scared to go to the doctor's or take a pregnancy test because of what it might tell me.
My main question is how long can you have a missed period before it becomes a "you missed you period because your pregnant" stage?
I have been a bit stressed out from school and other things (and now this, it's been freaking me out), and I have a bad diet. I don't sleep regularly (roughly 2-3 am to 9-11, depending on the day), and I know that your period doesn't become regular until you hit your twenties, but...I'm still really worried.