You’re in a union, talk to your union rep, bring documentation of the incidents and see what they think and if they can help. If they agree and help, great.
If they can’t help, then anytime anyone outside your management chain asks you to do something, you can say something to the effect of I’d love to help, can you just ask my manager for permission to prioritize your task over their normal priorities for me. If your manager/management chain asks you to do something, make sure you tell them what won’t get done properly or timely if you comply with their task “If I do XYZ tasks, then I won’t have time to finish ABC priority today”, if they’re ok it, then you document it and suck it up.
The keys here are: always act as if you’re willing and happy to help, you only do work authorized by the people who can give you work, the people who give you work are the bad guys if they say no and they become aware of all the extra requests of your time, don’t overload your trying to carry your own work and someone else’s, document as much as possible in case someone in your management chain has an issue with you not having done something that a manager agreed to.
That’s what’s worked for me in the corporate world at least, not sure what your environment is, so YMMV.
If you feel like what they’re doing is impacting your ability to do your assigned responsibilities, tell your manager. If you feel like they are going to bad mouth you to your manager, you can CYA and give your manager a heads-up. If you’re feeling personally harassed, that goes to HR, evidence is helpful. Otherwise, just continue to enforce your boundaries, you don’t need to explain the why to anyone not on your direct team or management chain, simple answers are fine: “Sorry, I can’t help with that.”, “I’m waiting for my next patient to assist”, “I’m taking my pause now and will be back in 30.”