My roommate called an ambulance for me and now I'm livid.
Why are you angry at your roommate for calling an ambulance?
He called an ambulance for me when I had a lot of things to do before I could go to bed.
If your roommate called an ambulance, it is possible that he thought you were having a medical emergency. If this is the case, then you should see that your roommate did the right thing out of concern for you. Instead of being angry, you should thank him for caring.
I hate him for calling the ambulance, I almost had to miss my plans.
If you were having a medical emergency then he could have saved your life. You should be grateful, because living longer is better than not missing your plans.