ive been scared of sex lately

i dont know why. absolutely no idea. but since last wednesday night ive been scared to have sex. i dont remember anything happening the time before that. and it isnt all the time. ill want to have sex til we actually go to start and then i just.........freeze. idk. like, sunday morning i got into it just fine. like, once we'd actually started i didnt have a problem. so why now?