My girlfriend sends nudes to another man

I've been in a relationship for 3 years. Yesterday I found messages and pictures my girlfriend had sent to another guy asking to meet up for sex. She had also sent naked pictures to him. I've I confronted her- she said she was sorry but then blamed me for not paying her attention due to my job. I work 6 nights a week in a gym. Is this cheating in a relationship? What should I do?