Constant hard stomach - 38w

Woke up this morning to a super hard stomach that hasn’t budged all day. Baby still feels head down, back still facing outwards so I don’t think position has changed but I feel unbelievably tight/uncomfortable and my stretch marks have gone from just about visible to deep purple over night so I know I’m not imagining the sudden tighteness.

It doesn’t come and go like braxton hicks, not constipated (if anything it’s been the opposite last 2 weeks), any ideas what it could be or is anyone else the same? I’m happy with movements etc nothing feels off as far as baby is concerned.