Antibiotics/stress Causing Late Period?

I really ought to be more educated on this sort of issue, but I am extremely paranoid. I am about 3 days late on my period, and I am regularly fairly regular. However, I feel that i've been pretty careful this month. The few times that I did have sex right after my last period had ended were all using a condom, and he withdrew before he finished, or did not finish at all. I don't think there were any accidents to the best of my knowledge. About halfway into the month, I developed a uti for the first time and took bactrim and another prescription for the pain for 3 or 4 days. After the uti symptoms went away, we had sex once with a condom and he did not finish. This one other incident did happen, however, when he finished on my upper front side. In the bathroom, I wiped myself off to where I felt it was all off. I think I was really tired however, and I seem to remember wiping myself once more (though at this point, there wasn't anything visibly on the paper, but i'm not sure if there may have been some that was not detectable?) and folded the paper over and wiped myself after I had been using the bathroom. What are the chances that this got me into trouble? There really shouldn't have been any contact at all, in theory, but I don't know if maybe there was an accident? Again, there really wasn't anything left on my front side that last time I wiped, but i'm not sure if the tiny amount that may have been there could have caused problems?

I am normally a pretty careful person, but I can't help but question what is going on with my late period. I'm not sure of exactly how probable it is that I am pregnant. I have been very stressed lately, but I really am stressed most of the time anyway! Also, does anyone know if an antibiotic such as bactrim can interfere with periods?

Thanks so much for the help, you guys. Any feedback would be appreciated...I am really worried.