Pregnancy with TTTS (twin/twin transfusion) is extremely risky, even after surgery. I am assuming you have only one fetus left after the surgery, meaning one of the twins is a demise? Depending on where the babies are, and where your placentas are, you can feel pain in your left side from something in your uterus, like contractions, abruption or even a cyst on your ovary. So even though your ultrasound was OK three days ago I encourage you to call your doctor and see if you can be seen for evaluation. Preterm labor is still a possibility, so if you still have pain and cannot see your doctor right away I recommend you go to the hospital for evaluation so you can be monitored for contractions. I hope this was helpful and good luck with your pregnancy.