Yeah, baby probably just changed position like back to back
Mine does that in fact it's quite squishy this morning. By tonight it will be firm again, you'll see!
I found it was more when I'd been laid back like chilling on the couch or sleeping on my back in bed that it was all soft, so I thought it might have been the back to back thing too, or just that baby was kinda further in towards my back if that makes any sense..! Try getting on all fours and maybe swinging your hips from side to side for a bit and see if it changes.