S.A.
Sounds like it might be eczema which I got near the end of my pregnancy, there are a couple of easy fixes for it - looks worse than it is but since there are also other options a trip to the Dr. sounds in order... it doesn't need to be your OBGYN - your regular Dr. should do!
Also might be a reaction to your anti-stretch cream since you don't have rash any other place... still a trip to the Dr. just to make sure!
Hope it clears up soon.
blessings,
Stacy