Long story: I was going to say muscle separation, but HotI beat me to it. I apparently got it from DS1 but only felt it with DS2. I didn't act on it straight away and it meant that my pelvis was unsupported (by those muscles) and i got SJD. That was ouchies too!!! So, i would recommend seeing a Physio, getting some advice and seeing if tubigrip (or pelvic support belt even) can help. As soon as i put my first tubigrip on i felt like somebody had just taken a huge weight off my tummy. And I was tiny too.

Short story: See a physio or similar, get some tubigrip. It's good for after the birth too.