I think I caught my dad 'cheating' on my stepmom - what should I do?

My stepmom was sick last night and was up in bed. I went downstairs to the kitchen at about 3 in the morning because I was thirsty. When I got downstairs, I saw my dad watching on the TV and talking dirty to someone. I just went straight back upstairs. I didn't confront him about it because I was too embarred and didn't know what his reaction would be. Is he having an affair? And if so, what should I do?