What Causes A Rash On Creases Of The Legs?

I am 29 + 4 weeks pregnant and have developed a rash in the creases of my legs. I have never had this problem before. The rash reacts differently to different underwear, for example it seemed to be getting better but after wearing lace pants yesterday it was sore again. Could this just be a heat rash or could it be something more sinister?