Do I have the right to be angry?

People say I'm a very angry person, but I wonder if sometimes I have the right to be angry.

My mates and I have been going to the gym for around 6 months together but I'm the only person with a day job. We have a set time of 2pm to go to the gym but that changes almost every day depending on a lot of different things. The thing that annoys me is that I'm always the one who has to force a set time from the two of them while they don't bother giving me the same courtesy even though they know that I have to move my schedule around for them. So today, I had enough. I said it was unfair, that we started going to the gym together to motivate and help each other but they're only willing to work around their own schedules and not mine. Maybe I should just take it as a hint that they don't want to go to the gym with me at all..