My Dad Is Cheating

Two days ago I realised my dad was having an affair. He was taking me to a shop and said he’d wait in the car. I went back to the car to tell him it might take long. As I approached the car I realised he was on the phone, but as soon as he saw me he hid his phone. When I went back to the car the second time, a van was hiding me so he didn’t see me until I got to the door and he did the same thing but with a lot more haste. My mum has suspected him before and they have had major arguements over it. He seemed to start being faithful again but this incident just started my own suspicions again. I cried myself to sleep over it last night because I hate the fact that he’s not happy with my mum. There’s nothing she doesn’t do for him. So what do I do? Do I ask him? Do I tell my mum? Or do I ignore it and hope it goes away?