I'm sorry if this is a stupid question

But I was wondering when you're 5 months pregnant and you lay on your belly are you actually "squishing" your baby? If not why does it hurt to do so? I'm going to be 21 weeks pregnant tomorrow and I was laying on my belly for literally like 5 minuets and now it hurts really badly :( I'm not going to do it again, I just out of nowhere started laying on my belly and didn't think anything of it since my bump is pretty small