Cramps near belly button and dizzy

I have been having cramps around my belly button for the past half hour and when I was standing (had only been standing for about 10 mins) I felt dizzy like I was gonna pass out. My heart started beating really fast and it quickly passed. Now I feel weird. Like weak and legs are jello. Still having the cramping. I am 18 weeks today. Advice? Should I call the on call at my drs office?