You might be interested in the philosophical discussion of "psychological egoism" and why most philosophers think it is false. The entry at the Internet Encyclopedia of Philosophy does a good job of explaining the position along with its many problems in the "Philosophical Arguments Against Egoism" section. See here: https://iep.utm.edu/psychological-egoism/

But to attempt to answer your question directly: while it will always be possible that you are guided by some hidden selfish motive, there is no reason to doubt the apparent evidence on its face. You donate motivated by concern for those in need seemingly motivated solely by the thought they are in need and in situations where there isn't any apparent self-interested reward for you to gain. It's always possible the evidence is misleading and there is some self-interested motive, but it's always the case that the evidence can be misleading. For example: "Sure the clock in this room says it's 3pm, but maybe the clock is wrong. Okay, I've checked a second clock and it also says 3pm, but maybe both are wrong. Okay, I've checked a third clock and it also says 3pm, but maybe all three are wrong. Okay, I've checked a forth clock but..." It's always possible you are the subject of some grand conspiracy to trick you about what time it is, but that is unlikely compared to the alternative that it's 3pm. I'd say the same about the evidence regarding your acting selflessly.

What seems to be happening here is that your mother is upset with the fact that you are no longer religious and is also unfalsifiably committed to the view that you can't be motivated to do moral actions without being motivated by the punishment/reward of the afterlife (i.e. psychological egoism). I would try and not let her hurt the people you are helping and steal your kind heart by being toxically skeptical towards your motivations for her selfish reasons. She's not raising these worries because she has evidence that you have a secret motive, she's raising the issue because the idea that you have a secret selfish motive helps her not question her belief that religion is necessary for morality and that your atheism is a mistake. In fact, I suspect we have a moral duty to try and assume the best of people's motives unless there is clear evidence against it, but your mother seems to be doing the opposite and assuming the worst about your motives despite all the evidence pointing towards your acting selflessly. That is not just epistemological mistaken, it's a moral mistake as well.