I have had the same thing since about 8 or 9 weeks too, and it is horrible. I once got stuck on the toilet in the middle of the night because I couldn't take any weight on my leg!
I was referred to a physio who didn't actually do much but did give me some really useful advice that has made a huge difference. It is generally so much better now after doing some simple things. I'mm 33 weeks now and for the last 6 weeks or so have only had a couple of bad days.
The advice is to avoid rotational force and pressure on the pelvis especially on the SI joints. If you can do that and let then inflammation settle it often eases it a lot.
So, don't do anything on one leg. When getting out of a chair, use both legs equally, don't try and get up with more weight on one side. In bed, lie on your side, so the weight os taken on the main pelvic bones, not your back. Swing your bosy round and sit up from the side, and then stand up using both legs.
When getting into the car sit sideways in the seat and then swivel round both legs at once, other way round to get out.
When going upstairs, take the weight on the good leg, do one step at a time.
Avoid carrying heavvy bags on one shoulder
Buy a maternity belt and wear it for a few days - they are deeply unsexy, like an old mans truss, but they do really stabilise the pelvis and ease the pain.
Simple stuff but it really did work!