Butter has gone rancid, I kept it in the door of the fridge & that's got a poor seal, so things aren't as cool there as they should be. Plus, I'd left out some clotted cream & it unclotted itself, stopped being cream and looked more like butter with tiny lumps in it.
Solution: add salt to the dodgy cream and beat it into butter. Job done, I don't have to head for the shops later just to get butter.