I'm late...

I'm only late four days but I'm starting to freak out. I mean who wouldn't? I am only 22 and am in no way ready to have a baby right now. I have been sexually active but always use a condom... The only thing is...I can't remember if I had sex after my last period. I have picked up kickboxing twice a week after not excercising for a long time and I know that that can delay your period...but I am very RARELY ever late. How do I not remember the last time I had sex?! This feeling is going to bother me.