n/c'd for this.
I've been with my company for a few years and enjoy working there. Recently been promoted and the work has been more varied and also busier but after an initial crazy period I'm enjoying the new challenge and getting things done in time while not feeling like I'm losing the plot (most of the time). However, things have changed over the past few months.
Our dept was a large one to begin with, but had merged with another much smaller dept. This included 'Daisy' a colleague who my predecessor, 'Polly', didn't like very much. Polly would complain to me about Daisy being 'too busy' to do certain things they were both responsible for and shifting work her way but said she had to be careful what she said or did as she felt Daisy might go to HR. When the depts had merged we had several discussions with our managers about who would do what roles (eg if we would be sharing work with others doing similar jobs) and eventually it was decided that we'd stay as we were. So there are several of us doing Tasks X, Y & Z for teams A, B & C within the new dept. I have always helped with teams A and B, while Daisy deals with Team C.
Things have been fine for a while but recently there was discussion about 'efficiencies' and it was suggested that Daisy and I could help with some of each others' workloads when we are on leave. Daisy was on leave recently and asked if I'd do her Task X when she was off. There was not much to do and I did it willingly and thought that would be it.
The following week Daisy was back and I got an email from someone in Daisy's Team C asking if I could do their Task X. Upon scrolling the email I saw they'd already asked Daisy and she'd said she was super busy and could they ask me instead. I was a bit 
about this but agreed as it was not yet my busy period. As it turned out this Task X was a lot more complicated than I'd realised and a lot more work was involved. Not sure if Daisy had realised this because the work had been started some months ago by someone and left. The colleague was stressing out about it not being done. I eventually got the work done but noticed this is happening more often with Daisy complaining about being "rushed off her feet" and telling colleagues in her team C to ask me to do it or colleagues in Daisy's team just coming straight to me asking me to do what is actually Daisy's work. So I've now been doing work for Team A, B and C. I try to be helpful I guess but I'm really busy myself and it's starting to worry me. I'm not even sure what Daisy does, it's a similar role but different. I don't think she's sitting doing nothing while I do her work but it seems to be a pattern of her passing work on as Polly experienced this too.
So things have come to a head today. I have had a super busy week anyway and today was just the worst. Another of Daisy's Team C people emailed me asking me to do their Task X. Having barely had enough time to get my own workload done (which also included stuff for Team C that Daisy is trained to do) I was really stressed out and worrying that a) Daisy and Team C is going to keep piling work on for me to do b) I'll get in trouble if I say anything and c) I am very busy myself and have deadlines to meet and I'm even more busy now so I'm getting stressed; I had a bit of a cry at my desk. Hopefully nobody noticed.
I was so stressed and incensed that I replied to the Team C person saying that I deal with Team A&B's Task X only, that Daisy deals with Team C's Task X so they should speak to her. He then replied having cc'd Daisy into the email saying that she's really busy and can I help out. I then got a very snooty email from Daisy herself, saying she was extremely busy doing Very Important Thing and she'd be much obliged if I'd be ever so helpful and do colleague's Task X. So I said I'd try to get it done but I was having a very busy week too and still had to meet all my own deadlines. Grrr.
I then constructed an email to my Manager about what had happened, detailing that although I had agreed to help with Daisy's work when she is on leave, nobody had said that I should be helping with her workload when she's at work as well and it's not in my job description. And that Manager knows how busy I am doing Task X as well as everything else and maybe it's a resourcing problem. I haven't sent it yet but I thought it was a good idea to keep a diary of this stuff. I'm also thinking I should meet with my manager to discuss it. I hate this, everything was going well too. :(
My manager is a nice person so I'm hoping she will be understanding. Also hoping that this won't snowball and I wont get in trouble or Daisy won't go to the manager/HR and complain about me not 'helping the department' or whatever. Part of me is also wondering if Daisy is one of the reasons that Polly jumped ship.
Any advice would be much appreciated.