I saw the physio last week so maybe can pass on some advice.
Have been getting back pain around my lower back / pelvis (no pain in front of pelvis / public area thank goodness)
Basically what happens is that when you are walking around you are aggravating the pain because your pelvic joints rub against each other and become inflamed.
The reason you notice it more after sitting or lying down is because the inflammation has flared up while you're resting, although resting itself is beneficial. When you get up again it eases off, but you are starting the cycle all over again by moving around and the joints rubbing together.
Sitting and lying is the only way to break the cycle - walking will only inflame it.
I've been told to stop walking completely if possible, which is difficult as I have a 20 min walk to and from work - have had to find a car share or work from home.
Cycling is better than walking, but cycling uphill puts pressure on your pelvis so still avoid as much as possible.
Other don'ts are standing on one leg for any reason (sit down to put on trousers or shoes etc), lifting or doing anything unevenly with one side of your body.
There are exercises you can do (pelvic floors, all fours pulling your tummy in for 5 secs, cat stretches) but they'll have no effect if you continue to walk a lot.
Swimming helps, and try a cushion in the small of the back when sitting to relieve pain. Also in bed, try to support yourself with a pillow underneath your bump, between knees and under legs. Best position to sleep is "recovery"-type position on side with lots of pillows.
Been given a support belt for when I have to walk and I've found it really helpful.
The most helpful thing has been a week's hols in Italy lounging by the pool last week
which has got rid of most of the pain and now I can concentrate on resting more and not letting it flare up again.
If you're really suffering then get a referral to the physio from your midwife - when you're pregnant you bypass the waiting lists so you can be seen within a week which is great as you can sometimes wait for months to see a physio normally - make the most of it and take care!