I'm 24 weeks, and everything is normal and healthy for me and bump. I had some mild right sided sacro-iliac joint pain (in laymens terms, this is where the spine and the back of the pelvis join as joint that has only a small amount of movement to it) early in pregnancy associated with something pre pregnancy that aggravated it - probably work related (NHS) or lots of pre pregnancy distance running, but never any true diagnosed back/joint problems.
I saw a obstetric friendly physio who did some manoeuvring of the joint and said the right side was stiff but really out of alignment, gave me some exercises to do and its all been OK.
The last 2-3ish weeks (perhaps longer) I've started to find when I've been sleeping in bed for a few hours and wake up, my sacro-iliac joint (now predominantly left side) is often - but not always stiff and a little painful. It almost always resolves once I've gotten up and walked around for a little while - to the loo and back usually suffices. On a few occasions I will be aware of some discomfort during the day, but its mostly when I sleep/get out of bed. I generally favour sleeping on my left, although I often wake up sleeping on my back. I have a long pregnancy pillow for bump and leg support which doesn't make much difference to the stiffness, nor does sleeping on my right side.
I'm going back to the physio to see what magic he can work again, but having never been pregnant, I wondered what other women's experience of sacro-iliac/pelvic pain has been. I don't really know if some stiffness is to be expected with ligaments softening. I'm well aware that some women have some horrendous pelvic pain that results in support belts, crutches etc etc and this is no where near that scale. I'm just interested to see what other peoples experience are in comparison.