hi pacha, i also have SPD and it sounds like thats what you are experiencing which isnt nice
SPD - Is the 3 sections of bone that make up your pelvis are held together by ligaments and these loosen ready for labour, however they can get a bit too loose and so cant cope as well with the stress of carrying baby hence the pain.
They recomend taking small steps when walking, keeping your knees together when getting up and down and also in out of a car, and taking it easy.
You will find it worse in the morning and evening and if you sit or stand for too long at a time, if you ask your MW she should refure you to physio and they may give you a back support etc...if this is what you have you have to make sure you tell them when you are in labour as then need to be careful of how far apart your legs go.
Anyway, the good news is that no matter how bad it gets i can speak from experience and say it goes virtually straight after you have baby
Hope this helps, there are a few of us on here that have it so plenty of people who understand howrotten it can be
you may find those who have not had it a little less than sympathetic as they think its just a bit of back ache but just ignore them
xx tigger xx