Trouble at t'mill

Well, I had my monthly meeting with my manager yesterday during which she said she wanted to give me an informal disciplinary warning. Apparently I am failing to meet any of the criteria on my job description! She also mentioned that I'd had bad references. This all came as rather a shock - as she had recently been praising my work and a few weeks ago I had an appraisal where she didn't bring up any concerns about my work. SO off I went to see the personnel representative who assured me that my references were excellent and we looked at various ways of dealing with this. I'm also off to see my union representative today, find out what they advise. But I'll probably have to have a big meeting with my manager next week to go over all this and put forward my case - something that I'm certainly not looking forward to...