I have that same thing! It started with what I thought was my bra (not underwire) irritating my rib on the left side. That went on intermittently, for several months, then seemingly out of the blue, it started hurting more and more. I will see a Rheumatologist in a couple of weeks and will get what I'm thinking will be a diagnosis of costochondritis.

Yes I get pain under my rib cage to and of I went to the doctor come to find out it was just part of the fibromyalgia

I was told that was from constipation and put on a medication to take care of it. It has helped.

My fibromyalgia has me reeling in pain in my left rib cage, low I think 2nd and bottom ribs of the rib cage, it's constant. Been worse the last year, even thought I fractured it at one point, x-rays showed clear.
God bless

I have it too. I think it's due to fibro.