18 weeks pregnant with menstrual type cramping in right ovary. What could this be?

For the past three days now my I have been having cramping that feels like I am going to start my period. It only hurts on my right side near my ovary. I've not had any bleeding but the pain is unnerving because it feels so much like I am going to start my period. I don't believe it is round ligament pain from stretching, it feels different from that. I have my next appointment to my doctor in 2 weeks. What could this be?