Am I being selfish about my son's visitations?

My sons mother asked if I could give up an upcoming weekend with his so that he can go to their family reunion. I said only if I can take have him for the 4th of July and take him to daycare the next day. She said she made plans already and that she would cancel them if I gave up one of my 2 hr visitation days as well. My response was no because I like seeing our kid too. She then asked if I wanted him tonight (friday) and again I said no because I need to work. I do not ask for a lot extra time from her. When I do she usually says no. I don’t like negotiating with her since she usually wants more. She says I’m asking for too much and being selfish making her give up her plans or not letting him see her family. Am I?