Uterus position

So. Last night. My husband and I were having, you know.  Well my uterus got all tight and hard. And I realized that I could feel where it stopped, still very far down. I can feel It even when it isn't tight. I'm almost 17 weeks. Why do you think my uterus is still very far down? It's barely above my pubic bone. This is my second pregnancy. It's so far down that I don't even have a bump. With my son, I was already a blimp by now. Any suggestions? OB is on vacation so I can't call him. My next appointment is in 2 weeks.