T.G.
I had that when I was pumping for my daughter; although my sharp shooting pains were usually at the beginning of the session, and the dull ache afterward. My lactation consultant thought it was probably a combination of a rather enthusiastic let-down response and the whole plumbing system not being used to doing its job - the swelling of the vessels storing the milk, followed by their relaxing afterward, only to re-swell again. I used warm washclothes for relief, and as simple as that is it worked great!
If it doesn't go away soon or gets worse, I'd definitely call my doctor. If it's an infection trying to get started, you'll want to nip it in the bud. Those *stink*! And should be outlawed! And if *I* ran the universe, THEY WOULD BE! ;-)