Hi Sallymarie
I had the exact question about a month ago in the third trimester thread and it is exactly like too much horseriding or too long on a weird bike seat. No-one had a answer for me but I asked a midwife who said it was normal. Unfortunately, I have had it on and off (more on though) for about 6 weeks now and the only thing that helps me is to sit down. One weekend I literally could not walk with the pain, but it improved. I still have it today and am 37 weeks! So I do sympathise- I know precisely what you are feeling!