If I just kept a woman from getting hurt, how come I have a lump in my stomach afterwards?

My neighbor (male) was yelling/arguing with a girl/woman on the other side of my duplex. It had escalated to a point where I thought he was going to start hitting her. I stayed locked in my side and called 911 on them, mainly him. I belive I did the right thing by using the safest way to prevent a woman from getting hurt. I did not get involved and let the police take care of it. So, why do I feel unwell afterwards?