I'm only 12 and taking care of a teacup sized chu wawa I don't know how old she is but she can walk play eat and climb I was playing my game and she was in my lap my mom came home and gave me some batteries she tossed them to me and acdentally hit the pup this happen last night and right now she has a bump I don't know if it was from something else or I don't know I'm scared please help!

The best thing to do would be to take the chihuahua to a vet to get checked over.

It sounds like this is a puppy, are you pet sitting or is this your puppy?
Chihuahua puppies are tiny and fragile.
Where is the bump? It's best to voice your concern to your parents and get her in to the vet to have her checked, whether you are pet sitting or it is your pup then it's the responsible thing to do.
If the bump is on her head, that is best done quickly since many chihuahua have open fontanels just like human babies.
An open fontanel is when the skull bones are not fully closed and sealed together leaving the delicate tissue underneath more exposed to injury.
Talk to your mom about taking her in.

Why would anyone leave a pup that tiny with a 12 year old is absolutely beyond me!