Trying to calm down

For some reason I have such a hard time being calm when someone changes plans on me or does not let know what is going on. I just found out that my plans for tonight are not going to happen because my boyfriend did not let me know that he had already made other plans. I am so mad and irritated with him. I never use to be this way. I use to be able to just go with the flow, but now it takes me so long to calm down and stop being angry. My heart is racing which is making me even more on edge. If I know that plans are going to change ahead of time then I am fine...its just this last minutes stuff that makes me so mad. I don't know if this is anxiety or if its just me being difficult, but I just wish I know how to stop feeling this way.