A baseball sized lump seems pretty big. Is there any redness? I'd be concerned about getting mastitis, which is an infected clogged duct, but there's a couple things you can do to help unclog it.
Use a warm heating pad or face cloth on the sore spot, press down on the outer part of the lump and apply pressure inward (towards your nipple), and pump pump pump!
I would see a doctor to be sure, but definitely get seen asap if you notice a red streak on the lump or if you start feeling like you have the flu (fever, aches, malaise).
Hope it clears up on its own for you though!