Some people are just evil

Yesterday, I had the worst day of my life on the checkouts - and it wasn't with a customer. I find customers are generally very nice and undersatanding.
My day started off as normal. Although I am only able to use one hand, the supermarket where I work still insist on putting me on tills where I find packing extremely difficult. However, I agreed with my manager that I would work on a less busy till and she was happy with that. The problem then came when another member of staff came in.
Orginally she was going to help the mamager do something on the shop floor but another colleague came and the manager then asked her instead which upset shall I call her the "fat blond" because she wanted a skive, like they all do. whereas I am happy to plod on. An hour into the shift and this "fat blond" wanted to do something else and this time the manager asked me to do it - bearing in mind this job was "no big deal" although it did mean that I would get off the tills for 15 minutes - which is what she wanted to do. So that upset her!
Then she starts on me saying why do I have to sit on this particular till; why can't I go on another? I told her thait my medical conditions were none of her business and told her to speak to the manager if she wasn't happy with what I did. Two fingers to her I thought.
I am normally a very placid person and would not entertain such pettiest but this "fat blond" seems to have a problem with everything.
I do the job to the best of my ability. My manager is happy with me and my work and leaves me to get on with things, so it is none of her business what I do and who is she to tell me what I can do and can't do.
I should have learned by now that most people who work on supermarket superstores do it because of a variety of reasons however, there are a few, like the "fat blond" who are not capable of working anywhere else and you can see why.

NB. I emphathise that this is not anit-fat people, I use the term "fat blond" simply as a way of describing her and nothing else. Fortunately, fat people like her are rare - just like people like her are rare. And unfortunately, we all need bad people to make us appreciate the good people in this life.