I think my dad is cheating on my mom, what do I do?

I could understand how you would think he is cheating on her and I am sorry you have to deal with this. I think your best bet is to talk to your dad about this if you guys have a good relationship. There could be a very logical explanation to all of this and you don't want to hurt your mom for no reason. Good people do stupid things like cheat sometimes and not just because they feel like it but because of some underlying reason they just haven't figured out how to deal with, remember that when confronting your dad, don't be abrasive, just caring, and hopefully whatever is going on, he and your mom can work it out.