I am wondering when you shouldn't lay on your tummy during pregnancy? Will I hurt the baby by laying on my tummy?

You won't be able to lay on your tummy much longer. LOL

Also, it occludes (closes) major blood vessels that supply blood and nutrients to baby.

I'm 18 weeks and I am just now noticing a little pressure in my belly when I'm on my tummy, so I normally just hike a leg up to give me a little room. But you're body will tell you. If you're asking, that means you probably feel it, which means it's time to get a pregnant pillow or start sleeping on your sides/back. Actually, I heard sleeping flat on your back isn't good either. But I'm not sure. Good luck!

You may want to avoid also sleeping flat on your back. As the weight from the baby can put pressure on a major vessel and can cause light headedness and a drop in blood pressure. The best way to lay is on your left side if you can.