My pup fell down the stairs and now has large lump on her neck. Help??

[Anars]

Yesterday afternoon(7/8), my 1 year old pitbull-bulldog mix pup, fell down my basement stairs(wooden, no carpet, concrete bottom). She was trying to run and hide because of the fireworks for the holiday. She got too excited, leaped head first off the top step, her body hit at about halfway down and then tumbled the rest of the way down. She is not limping, and only has minor scrapes. My major concern and worry, is the baseball size lump on her neck below her jaw on the right side, that formed within hours after her accident. She's still eating and drinking like normal, definitely more lathargic, shows no interest in playtime. Help me...please...any knowledge, advice or recommendations that one may have based upon my description, will be very much appreciated. Please help! Thank you