OMG, the hot doctor... it gets worst and worst with time it seems. Being 28, this is becoming more and more frequent when you encounter the ones that just seems to be done with their studies and are the same age as you... Kinda mess with my brain sometimes xD.
I would dig in direction of sacroilliac issues. Mine are still tolerable but I hate when it happens... sometimes its dull, sometimes it's kinda intense... As Nims said, a rheumy could be quite useful with these problems.
In all cases, before jumping to medication, if ever it is directing you towards these kind of diagnosis (arthritis of some sort /joint inflammation), I would tell you encourage you to look into physical therapy at first.
How would you qualify your abs strength? Mine, since surgery (I don't have my colon either, still have a rectal stump though), were pretty neglected. I've started working on them in the past month and so far it helped me a lot.
Do you see a pattern in your pain? Is it the same all the time or is it worst at certain time during the month? Also, would you describe your general flexibility to be superior to "miss and mister everybody"?