irritable uterus + contractions (21 weeks)

Over the weekend I started to feel what I thought were Braxton hicks contractions, by Tuesday they were really consistent, one after another.  They weren't painful just noticeable! The advice nurse told me to go into L&D so I did and they monitored me for a few hours, turns out I was having contractions! Not big just consistent.  They gave me a shot to stop the contractions and did an us to check cervix, which is still at a good length. They said they probably weren't true contractions because there was no pain, but more of an irritated uterus! Since then I feel these "contractions" on and off throughout the day.  My dr told me to not worry about going into L&D even if they are consistent, just to go in if they get painful! She said to just relax and drink lots of water when I feel them.  Anyone have an irritable uterus or have contractions this early on and still go full term?  I'm currently 21 weeks + 2 with twins...