Lately after sex i've had to pee really badly why could this be happening or is it just because i'm pregnant ?

Frequent urination: can occur during pregnancy specially as the uterus gets larger and puts pressure on the bladder, however, more importantly your doctor should check your urine for urinary tract infection which needs treatment with antibiotics if present

Because you're pregnant.: Two things happen. One is you get more pelvic congestion. The other is that as the uterus enlarges it puts more pressure on the bladder so that you really can't hold as much. It doesn't take too much stimulation to jiggle things around and make you feel like you have to pass the urine.