I once worked for a horrible company that treated employees terribly. I even once had a disciplinary interview for being out on my day off (because something had gone wrong and the person on call had not been able to fix it).
I'm glad I was finally given the boot though it was painful at the time ... I had to take them to unfair dismissal to get compensation and the pay they owed me.
The worst part in situations like this is that you begin to think it maybe is something you have or haven't done that is the problem when the reality is you are doing a damn good job and they are lucky to have you. Some people just like being nasty to their employees.