Stomach tightening at 12 weeks??

OK girls..so I'm 12 weeks with twins..this is my 4th pregnancy, I have 3 other children. Im a little confused because I never experienced this with my other pregnancies this early, I was always late into the 2nd trimester when it started...but since late Friday night Ive been feeling my stomach getting tight and hard..its nothing regular, and its happened maybe 4 or 5 times since Friday night. But it almost feels like mild Braxton Hicks contractions... nothing painful, and it doesn't last long, but it kinda makes me nervous. I can't get thru to my doctor cus its Sunday, and I dont want to go to the hospital if its nothing to worry about...so I'm just wondering if its happening because this is my 4th pregnancy, and I'm carrying twins???..has any other mommy's gone thru this? I went to my checkup last Wednesday, the doctor was able to pick up 2 heartbeats, they were very faint though, but my doctor said 1 was coming in at 158, the other at 161. she said they were probably just still pretty small, I was 11 weeks 3 days, so she wasn't worried. but now this new symptom is worrying me!!