I think my dad is cheating on my mom? HELP!?

Okay so awhile back by dad started complaining about me and my brother arguing. Yes we argue but not ALL the time and it's only sibling bicker. He would always say things like 'I always have to come home to THIS!' or 'just SHUT UP! I don't want to hear you talk.' etc. At first it was just annoying but then eventually he started always being grumpy! He would always argues with my mom (he starts it over nothing), and would just not be fun to be around, REALLY unpleasant. Then he started coming home late calling and saying he had to work/his boss is making him to whatever. Blah Blah, at first i believed him but it's every night and i can tell he's lying about something. Then eventually he started coming home on time again but there still was something going on. I could tell. He just kinda walked around, didn't really talk much. Then after about a little less then a month of that he went away on a business trip to Ottawa. He was there for 3 weeks. When he came back he was SOOO happy and in a good mood it was unbelievable. I thought he was over whatever was going on and everything was back to normal. But then he still wasn't talking to my mom. He comes home late every night, and i mean EVERY NIGHT all happy and joyful when he HATES his job. He's been on so many 'business trips' lately it's crazy, and whenever me and my mom will go out somewhere we'll say i'm go to _____ be back in a little. We will get back and he'll be gone! He will say he's at the grocery store, etc, he's even now been saying he was shopping! HE HATES SHOPPING! i don't know what to do! I don't know if he's cheating or if he's doing drugs or what! It sounds crazy because my dad used to normal ... im so confused. What do you think?