In the midst of folk saying you're catastrophising, I'm actually going to tell you you're absolutely not.
Do not attend this meeting. It needs to be rescheduled. As someone mentioned upthread, you should have been given appropriate notice - and scheduling this whilst you're on annual leave is NOT appropriate notice. 2 days isn't even appropriate notice - you would need to be given ample time to be accompanied, and this is unlikely to be possible (certainly with a union rep) in 48 hours. Preparing for a meeting that has been scheduled for the day you return to work from annual leave, without prior notice, would mean you'd need to be 'working' whilst on annual leave. Whether you are known to do this (checking mails etc) during leave or not, it's inappropriate to expect you to do so on any occasion, and more so, to prepare for this meeting whilst you're on leave. So it's either been done with the intention you are completely unprepared and are unable to be, or be accompanied, or, I guess, it could be good news.
You need to be accompanied by either a union representative, or a trusted colleague who can take notes from the meeting with you.
I sincerely hope it's actually good news. And it could well be, so do not panic. Unfortunately though, I speak from the experience of someone who was scheduled and tricked into a meeting with a manager and director of the company whilst signed off sick in the middle of a mental breakdown. Taking a screenshot of the teams meeting titled (and indicative) of a meeting about something completely different to what it actually was, as well as making notes of everything said in the meeting, certainly helped my case, is all I can say.
They should have given you some indication of the nature of this meeting. 'Catch up' is NOT it, when this is not the usual thing for you in your workplace.
You are well within your rights to question it. Do so.
Only you will have the intuition based on how it's been going in the workplace as to whether this meeting is likely to be positive or negative, and I'd base your behaviour on that. If you believe it's likely to be positive - great! But do prepare yourself with the opportunity to take notes, take screenshots of the scheduled meeting now, and be prepared. An interesting note was that in my case, the employer changed the title of the meeting on teams post-meeting taking place, to something more akin to what actually happened during it, thinking they were being clever and it would show I had chosen to attend the meeting unaccompanied (as they later tried to claim). They didn't know I had already taken the screenshot, which blew that out of the water, of course.