Has my employer and union broken GDPR?

I am in an ongoing complaint procedure with my employer which is not based on this point but it has been discussed.
I sent a lengthy email to my Union requesting legal aid because my employer was protracting my return to work process following a long period of sick leave. When chasing up the Union I was informed by a rep not dealing with the case directly that another Union rep was speaking to HR about my return to work.
My reaction was confused as I had not asked for this. I wished to be consulted about legal recourse as per my email to them. I reluctantly replied by text "Ok that's good", as to acknowledge at least they were doing something. But my genuine feeling was annoyance as they had acted outside of their remit. I had not expressly asked them to speak to my employer.
I queried the HR manager as to whether he and the Union had discussed my situation in accordance with GDPR. When pressed he stated yes. My main complaint with my employer is not weighted on this point but I am annoyed that matters were discussed that have had influence on proceedings without me present and no documentation to prove/disprove what was said.
Just to reiterate I made no explicit request for the Union to speak to HR and the notification of it happening was made by text on the notion that it was in motion or happening soon. I felt hijacked.
I hope this isn't too long winded or vague. I just want a rough idea of whether or not this is worth getting professional clarification and pursuing.