It's all down to very bad management, it has nothing to do with the colleagues
Some people are just prejudiced against people with disabilities. Whatever management did, these ableist colleagues might still think OP is being slack.
There is a difference between long term sick and long term disability. We don't know in OPs case which it is other than reference to feeling tired in the afternoons.
OP also mentioned she has some issues with her arms. I would say if an illness has gone on long enough and/or limits what the person is able to do markedly, then it is a disability.
come to think of it since the OP’s pay covers weekends what’s wrong with having OP work half the weekends and another colleague work another half?
@LateAtTate OP has said that for some reason her employers don't want to do that, they want people who are going to do the whole day at weekends.
Do your colleagues think you should either be fired or work till you drop to the floor? Maybe worth asking them that to their face the next time they make a mean remark
as long as you are prepared to get a truthful answer. Not everybody is diplomatic!
If anyone says they think OP should be sacked because she has a disability, or should work until she collapsed/without her disability being considered, that would definitely be grounds for a disciplinary.