My periods is late, hopefully no pregnancy! help!?

so, my period usually occurs on the 20somethings of each month. last month, it started like, the 24th, and i was done by the 30th. but, its the 26th, and i am not in my period now. on saturday, it started, then stopped. however, i am ually active. my boyfriend and i first had the 17th. (im going to explain this in detail)... we were messing around, and then we were talking, and decided to try , but only for a second. i licked any pre off, and then he... put it in. then took it out a second later. we rolled around, and did the same thing. so we had for a total of 2 seconds, unprotected. [i dont know if pre contains sperm or not, but ive been doing reserch on the internet, and i dont think it does, unless its after a previous ejaculation, which i know didnt happen withing the 3 days before the 17th. so there was no leftover sperm. and i got the pre off.] but im so stressed out that i might be pregnant, because of the late periods. my arent sore or anything. i peed alot of two days, but i also drank probably gallons of water those days, and havent been eating much sugar (which retains water). im currently dieting, so that has changed, which is also a role in the timing of your period, and ive been trying to get more exercize. but still, ive been getting signs my period is coming (for me, its belly aches, cravings, and acne. sometimes other things, but thats primarly it, and theyre all currently (still!) happening, and have been for like, a week). ive been sleeping alot lately, but since i had mono within the past year, and get sick easily, i think thats just colds and such (i get stuffy and my throat hurts, along with a headache too.) ive been so extremely stressed out though, even more than usual. i had midterms/finals, and have taken about 4 hour and a half-each tests in college chemistry. theres also stress of getting new cles (we do block schedling), and even extra activities (mock trial and speech, brain-tougher cles).[ and thats not even including the stress of thinking i was pregnant, which would bring me to tears some nights.] but the big question is, what do you think? i have a friend who told me today that she was stressed like this the first time she had , and her period was on-and-off for 10 days, but the next month, she was back to totally normal. i told my mom that my period was off and everything else, but the thing. i dont know what to do, and im trying not to stress, to see if that will change anything. also, i got my last hpv shot the 21st... i dont know if that matters.