when i had sex for the first time it messed my cycles up too, i was scared out of my mind and i thought for sure i was pregnant, i would have put it on my life that i was, but i wasn't. i was a week late, took a few tests and it said negative, my period came a few days later.
this has also happened to a few girls i have talked to before.
go take a test, if you are pregnant it should show up positive by now, i would think. but i doubt you are, good luck.