That's a really helpful link, puddock.
At 26 weeks (I'm 30 now), I got some quite bad pain at the sides of my hips, and across the back (sacro iliac area) fairly suddenly, after going for a walk - after having been v lucky up til then and had no aches or pains to speak of at all. I didn't think it was SPD, because everything I'd read about that described pain in the pubic arch, and my pain was all sides and back.
It was bad enough to make me walk very carefully, and I was worried this was what the rest of pregnancy was going to be like...
Anyway - saw my doctor, who gave me paracetamol, and said referral to a physio on the NHS would take so long, I'd've had my baby and then some by the time I was seen! - so I made 2 appointments with a private chiropractor. Cost me £50 for the 1st one, and £35 for the second one. So not cheap, but I was willing to try anything tbh. She said that you get surges of a hormone (relaxin?) at about 12 weeks, 26 weeks and... forgot the other one, sorry. Anyway, after these is when any pelvic/hip imbalance might appear - everyone is a bit lopsided one way or another, but it's normally such a small issue it doesn't cause problems. It's only when things get out of alignment beyond a certain degree that you experience pain etc.
Solutions - the chiropractor did some very gentle manipulation of hips and sacro iliac joint which helped a huge amount, I felt much better afterwards (was a bit cynical about it beforehand tbh, but happy to eat my words). She also recommended using frozen peas on lower back/sacro iliac area if I had another acute sudden episode - change sitting positions frequently etc.
I've been fine since then. Used frozen peas twice after walks that turned out longer than I was expecting, but that's it, and I feel very lucky it's been manageable.