I wouldn't worry too much more than likely it's stress that is delaying your period - just try to relax - by the way you don't have to have a period in order to ovulate so that's one bright side to look at . Just give your body time to regulate it's self but if your really all that worried...
I don't know what to tell you , you very well maybe pregnant but might not have enough of the pregnancy hormone in your urine for it to register on a pregnancy test - I would definitely go to the Dr and see what's going on .
Way too early to test - wait until your @ least a week late for your period. If your test comes back negative after a week after your missed period, wait one more week and then test again.
Honey if your not on birth control why are you having unprotected sex if your not trying to get pregnant. Your belly being really sore is more than likely from your period .