Braxton Hicks really shouldnt hurt, I had them early on (about 22 wks) but I could easily have mistaken them for baby stretching if it werent my third pregnancy so I know how they feel, I'd never had them before 38/39wks before either. If they are painful then you maybe should be checked out by your doc/midwife.
I think u should let ur Dr know. I have heard of ppl starting to have them at 20 week but not sure of that early. I had them early but my pregnancy turned out as a MC. At 20 week so just to be safe let ur Dr know. Wish u the best and may God bless you and ur baby