I don't think it's likely that you overdosed the vitamins, but if she already had kidney or liver issues, then possibly it could exacerbate that. Many of the supplements for poultry are very helpful in the short term, but not really intended (or good for) longer term use. It's very hard to say for sure what's going on. I think it's more likely that it's age related and she may just be failing. I know that isn't what you want to hear, and I certainly can't tell you that with certainty. Honestly, I would stop them at this point and just give her fresh, clean water. Feed her healthy things that she will eat, try to entice her to eat her regular feed. Sometimes mixing a moistened feed with a little scrambled egg mixed in will get them eating. No one wants to think about it, but if you do lose her, I'd recommend that you consider a necropsy. It may give you peace of mind to know exactly what happened, and it can provide valuable information in some cases for any other birds you have going forward.