Could also be a dis-articulated rib - where the rib pops off the sternum or the spine where it sits.
this happened to me - i got it from BF of all things, i think that i was lying in an akward spot... Anyway, i happened to be on a course run by an osteo that day and she popped it back in (hurt but the relief was almost instant.)
Pain was exactly as you discribed, also hurt to move my arm.

Unfortunatly there is no treatment for a broken rib.
I would go and see someone, a physio may be a good start of you have never seen an osteo or chiro before - or go see a Dr. If you have broken a rib you could be at risk of doing further damage if it is near your lung...

HTH and you feel better soon