I kicked my girlfriend out after a pizza delivery revealed she's cheating me

I lived with my girlfriend and one day she told me she was working late. That night I ordered pizza and when the delivery guy arrived he seemed confused and said he had already delivered a big order (two pizzas, drinks, dessert) to my address about an hour earlier, but I hadn’t been home. She was. I didn’t confront her immediately but it felt off, so I checked our security camera the next day and saw a man I didn’t recognize entering our apartment around 8 PM and leaving with her around 11 PM. When I confronted her, she admitted she had been seeing a coworker for months. I ended things right away, but now her friends are saying I’m an AH for going behind her back and not trusting her explanation. I feel like I just put the pieces together and reacted to what I saw.