I ended up calling the OB at 5am, because the pain and worry was keeping me awake, and she told me that:

- since the pain wasn't varying, but constant and
- since I wasn't bleeding or anything

it was probably OK. I gather that varying pain could be contractions (and that, at 24 weeks, would be really bad), and bleeding isn't good at any time, is it.

She reckoned it might be back pain radiating forward so it felt like period pain + back pain. I dunno. She told me to take some panadol and sleep.

I feel much better this morning. So whatever it was, it presumably wasn't anything very bad, though I was certainly worried in the middle of the night.