Big baby, small uterus?

Trinity • Im happily married and I'm on my third and final baby. We are happy to welcome baby Luna in April.
Has anyone else had this issue? Ever since 26 weeks or so, my baby has measured weeks ahead of her due date. At 30 weeks though, my uterus measured 28 weeks. I was monitored and sent home. 
I'm 32 weeks and a day now. My uterus measured 31 weeks, and due to various other complications such as cramping and diarrhea, I was sent to be monitored further. They did an ultrasound and my baby was measuring to 33 weeks and 4 1/2 pounds. They also couldn't find my cervix on the ultrasound. Now they want me to come back next Tuesday for additional monitoring and another ultrasound. They didn't elaborate beyond that. Did anyone else have this issue?