New abdominal discomfort - round ligament pain??

course11

As background, I've had a very uneventful pregnancy and mild symptoms so far. Baby looks healthy and I'm not high risk other than my age (38). I'm currently 21w2d. Around 7pm last night, I started having a weird stomach discomfort that I had a hard time describing to my husband, and I'll probably have a hard time describing here.

It's not cramps or nausea, it's not the baby kicking, and it's not accompanied by any other symptoms (discharge, spotting, etc). It's not a sharp pain, nor is it localized to one side. It's more like my whole stomach area feels like it's getting stretched out all at once. On a pain scale of 1-10 it's probably a 3. But it woke me up at 3:45am and I don't know what to make of it. Is this some strange version of round ligament pain? Is the baby having a growth spurt that's causing an unexpectedly fast stretching of everything? Has anyone else experienced anything like it? Thanks for any insight or experience you can share!