Contractions?

I'm 27 weeks today and I think I'm having a second contraction.. I have Braxton hicks from time to time and they never hurt. But I've had a very crampy feeling like a bad period cramp and it starts not as painful then more painful and I feel a pain shooting down my left leg which I sometimes get with my period cramps is leg pain.. when do I worry and go in? Do I just let this pass and then if it keep happening go in? This is either the second one in 5 min or the same one that's lasted the whole time..