why can't I shake this feeling. HELP

I have days that I just am anxious and get scared thinking just to much on what happened awhile ago. I know you can't be pregnant with sperm aka cum on your pants or panties but why am I still freaking about it. I see a therapist for my anxiety but why can't I shake it. I have all texts pointing to not
I would be as it happened in dec week 1 2016. So what is the issue. Why is it so worryingly to me. Just please kind words help me.