“My son hit his head and has a big bump while staying with my mother, but she won’t tell me what happened. I took him to the doctor and he’s fine, but now i don’t know if I can trust my mom to watch him, should I still trust her?”

This is a tricky situation for you. I am sorry about it.
I guess your son is too little to tell you what happened, so you can only rely on your mum and she won’t say anything. As tough as it is, your priority should be your son’s safety, so if I were you, I would tell my mum that she cannot spend time alone with him, unless she tells you what happened. I would tell her that we are happy to hang out with her but that right now I don’t feel safe leaving my son with her.
How is your mum’s health? Could it be that she is not telling you because she does not remember? Perhaps it is worth taking your mum to the doctor for a checkup.
I wish you all the very best.
Love,
Ana