@Elisemum in this context it's not JUST rudeness if they were disparaging comments about op personally and her health.
That's disability discrimination and illegal and absolutely part of HR job to deal with it (but remember HR are employed by the company op, they exist to prevent them from being sued not primarily to support employees)
They are within their rights to complain about their workload and the company's failure to ensure not too much is being asked of them, they are not right to comment on op and her health issues.
Disability comes in many shapes and forms, chronic illnesses with fatigue as a symptom are very common actually and are absolutely included in disability at work legislation.
People need to learn to be more understanding and direct any frustrations regarding the impact on themselves at work due to such situations where it should be directed - at the company.
If there's too much work for the number of employees/hours being covered that's on them for not employing an additional employee or giving more hours to another part timer who would want to perhaps go full time.
A lot of companies don't want to pay out for recruitment, training etc while the execs pocket a nice stash of bonuses!
Point your ire where it belongs - not at the sick/disabled person but at bad employers