Thanks everyone!

Yogababy - Thanks for your helpful advise (as usual!), I Googled SPD and it definitely sounds like that's what the problem is. I have a pre-existing problem with my pelvis (it is very unstable) due to a fall at work years ago so it is likely to be caused by that.

All Girls - Thanks for your tips too! I had one of those braces from last pregnancy, but found it makes the pain worse . I will definitely be using the heat pack tonight.

Kellie - Sorry to hear you are suffering too!