sounds like SPD (PGP), you're lucky to only be getting it now!
try not to do anything that makes it sore (easier said than done!), I found that special pillow helped (dream genie??) and saw a physio but you might not want to get a pillow this late on and it might take a while to see a physio? speak to your midwife if you can!
hopefully madam comes early so you're not suffereing too long, it's a total pain in the foof!!