diaphragm pain?

[royal_penguin]

I've had this dull ache/pressure in my lower ribs area, right under the breasts. Including the top of my pregnant belly. I can't figure out what exactly hurts. It gets worse when I'm sitting. When I lay down it gets better. In fact, I have almost no discomfort when I get up in the morning, but as the day progresses it gets worse and worse, especially when I have to sit for a while. I try to straighten my back as much as possible when I sit to relieve the pressure but it hardly helps.
At first I though it was pressure from the bra so I bought those wireless nursing bras but they didn't help any.
I am almost 29 weeks, by the way.
Has anyone experienced a similar problem? Did anything help relieve the pain?