Pregnant with twins

I'm currently pregnant with twins I'm 8 weeks 6 days and i have been having a lot of cramping and it's concerning me. This is my 3rd and 4th baby my first two were born via C section and my youngest is 14 months. Other mommas that have carried twins did you have cramping? Also at my ultrasound my doctor said he couldn't see the membrane in between them and I have to have another ultrasound at my 12 week appointment to have another ultrasound and if he still doesn't see the membrane in between them then he wants me to have a special ultrasound done, had anyone had this happen? Sorry for this being so long I've had losses in the past and I'm pretty worried about loosing our babies :/ Also sorry if I posted this in the wrong section.