Thanks MN & Sparklez. I have no idea what it was but it made for a truly horrible day. The midwife reckoned it sounded muscular and to call back if the pain went around the front too, but that never happened.

I was sick (think I may have overheated when I had a shower) so DH got pretty worried. Other than that I finally managed to sleep with a heat pack and haven't had a pain in my side for a couple of hours, hooray! On the plus side I'm a bit crampy at the front again so hopefully this all goes somewhere one day