Girls, hi. Maybe someone has encountered such a problem. I'm 19 weeks …

Girls, hi. Maybe someone has encountered such a problem. I'm 19 weeks pregnant, a couple of days ago my belly became firmer than usual, it's most noticeable when lying on my back, I think this is hypertonus, but I don't understand from what. There are no suspicious discharge or pain, I have a scheduled ultrasound on December 7th, should I rush to the doctor?