Wow, that is really harsh. I'm so sorry you've had to go through this. The CAB will be able to help. I've had problems with the DWP (not this, but related) and they can be incredibly snotty and unfair. They made me feel like a benefit cheat when I was extremely unwell. I do know how you feel. Just be prepared for a bit of a battle; that is my experience, anyway.
This is not your fault. This is THEIR problem; not yours. It is wrong and unfair and is IS discrimination. You have every right to be angry. Use that anger as motivation. Find any paperwork relating to your benefits and any letters you might have supporting your claim to bring with you tomorrow. Good luck and please try not to beat yourself up over this. It's not your fault.