Baby Time?

I'm 29 weeks, almost 30 this coming Wednesday (4 days away). I went to my apt and they said we were measuring 3 weeks ahead! I've started having Braxton hicks terribly since yesterday!! We go back January 30th to get an ultrasound to see how big our little girl is and then they'll decide from there what the plan is... So I'm thinking our once March baby will now be a February baby. I'm getting more nervous and concerned with these pains.. so I thought I'd ask mommas on here who have been pregnant before, how do you know the difference between Braxton hicks and the real deal? Thanks for any help and reading my story. 💕