On top sex

So I was on top bc it’s usually the most comfortable position but tonight while I was on top my lower back and pelvis was hurting while I was “riding” him. It was kind of painful... but when I leaned down on top of him it stopped... anyone experience this?! Just a little worried that it’s a bad thing or maybe my lower back is just screwed up with the expanding baby bump... just hoping I haven’t hurt anything bc I kept going for maybe a couple minutes thinking it may stop before I changed positions... lower back isn’t hurting now but def can tell it’s been used if that makes sense....