Pain/pressure in sternum?

So I'm 30 weeks and just starting today I have been experiencing discomfort/pain in my sternum area, right below my boobs. it feels like someone is standing on my chest and making it hard to breathe. at first I thought it was the underwire in my bra, but I took it off and it didn't go away. I've googled it and some women say it heartburn and others say it could be my gallbladder or just my organs being squished together to make room for baby. I've had heartburn this entire pregnancy and never had this kind of pain. It did start right after I got done eating though. any idea on what it could be?