Try not to worry.
It sounds like your milk glands are swelling. Thats very normal. It happened me when i was pg. and my golden retriever got the lump in the armpit thing. It went away when she stopped nursing :-)
Mine was noticable when it was time to feed the baby but not after i was less full.
Being pregnant is such a wonderful time try to enjoy it. Once its over then you have to share the baby right now he/she is all yours.
Think lots of happy thoughts.