I also got that last night and it scared me as i thought it was my heart or that i had cracked a rib. it was hurting even more when I breathed deeply. anyway i got home and took off my bra and found it had perhaps been too tight? had a hot shower and DH gave me a massage on my back (back was also sore but i do have a bad back) and it went away totally...today bra is on looser and i am trying v v hard not to slouch etc.
Could be a good excuse to start getting massages regularly!!!