M0nica
Well, immediately join your work union and you will have help, assistance and support
As an ex Union rep I have to say that no, joining now will not help with a problem that has arisen before joining. Unions are very helpful to members, but people can't join them when a problem has already arisen, any more than you can take out house insurance after your house has caught fire and expect to make a claim. Many people don't want to pay the subs, or have to take collective action, but expect protection when they need it, and it doesn't work like that. It is definitely worth becoming a member going forward, but it is unlikely that the union will give support with this meeting, although if it is a disciplinary meeting there should be a right to take a colleague along for support.
I sympathise, Bea - your situation is clearly untenable. Does your house insurance have legal advice as an add-on? Alternatively, check if a local university has a law clinic that will offer the services of a supervised student who can advise you. They will have an experienced lecturer checking every stage, so you will be getting sound advice, which will count towards the student's qualification.
As others have said though, there is no saying that this will be disciplinary - it has a good chance to be a routine 'back to work' meeting as your employer will have a duty of care to ensure that you are not being asked to do more than you are capable of doing. Good luck.