What Causes Delay In Periods?

period is 11 days late, had unprotected sex twice about a month ago. i took a pregnancy test about 4 days ago and it was negative (possibly took it wrong?) . i am an epileptic currently taking trileptal and keppra. i had bronchitis earlier this month and they gave me anti-biotics (starts with a z?) what is wrong with me?? im 17