It sounds like your sacro-illiac joints and sacrum are loosening in preparation for the birth, and it is your hormones that are responsible for the softening. This is something that happens to every pregnant body and it is very easy to over-do it and hurt yourself, speaking from experience!

The pelvic joints are held together by ligaments and it is a good idea to stretch and strengthen these mucsles, so see if you can get a referral to see your local hospital physiotherapist so they can show you how. I found out after 2 weeks of limping around in pain (especially after standing up) that I could have got a referral to see a physio, for free too.