Heya sweet
I'm suffering from what sounds like the exact same thing as you!!!
my pain is low down the back of my spine. I think its part of the pelvic bone. seems to be only one the right side of it (so far)
I think the hormones that relax all the muscles and ligaments have something to do with it!
Ive used a heatpack like you and am yet to try any lotions.
I read on a website the reason for it is that the hormones move the pelvic bone out of its normal position.like with other joints too. Making sure your sitting upright and walking upright is the best thing for it....hopefully it works!!! Ive got one of those gym balls that have been giving me some relief
Keep me posted to see how long it took you to feel better. X