Also, honey, you really are awfully young to be shouldering the responsibility of all this worry and stress. Why do you need to be having sex at 15? No guy is worth this kind of worry, and no guy worth his salt would insist a girlfriend aged 15 should have sex.
your period could be a little different because you had sex for the first time. if you want to be really safe guarded against pregnancy, you should check out the birth control pill + use condoms :-) good luck