I have had aching ribs for the past month. When I am standing it is fine but it really causes me grief when I am sitting down. I dont think it is because bubs is that big that he is pushing up on my ribs... I think it has more to do with me sitting down all day at my desk. I dont have the best posture anyway eek!
I find getting up for a walk to the toilet or to the water cooler every half hours makes it a little better. Or putting my arms up against the wall, above my head and breathing in.

Hope this helps a little.