It sounds like round ligament pain to me. This usually doesn't show up until a little later, but since you are carrying twins, it would probably happen earlier. It is the ligaments that support the uterus stretching as the uterus grows.
But, by all means, mention it to your doctor at your next visit, or give the office a call and see if they would like you to come in and have it checked out more closely.
Congratulations and all the best!