Here's another thread with pec minor discussion & info
http://neurotalk.psychcentral.com/thread164739.html

I would stop any weight work untill you get this figured out.
It sounds like a strain w/some myofascial adhiesions going on after the fact.

slow steady posture work and Sharon Butler's book and/or website will have many ideas for you to try. Her book is in most libraries, but many articles on her website.
www.selfcare4rsi.com

Delayed pain is a TOS like thing, happens often.

Check out our Useful sticky threads above the main thread list , there is a wealth of info on all RSI/TOS type things. therapy and selfcare too.