Thanks a lot for all of your feedback. I had to do some digging to find some of the information. As far as I can tell from about a couple of years of bills, there's a pretty wide range of monthly water costs from about $300/month to $1,500/month. The month in question was about $2,400. And yes, they are saying that I need to pay the amount over the normal monthly water cost.
I read the HOA legal documents and they indicate that since there's a single meter for the whole building, it's a shared common expense amongst all of the owners. I don't know if this is enough to say that they can't charge me anything beyond what I already pay in HOA fees because it's a common expense? Maybe I need to talk to someone to get some legal advice.
PS, I don't know why they didn't put in separate water meters for each unit at the beginning...this seems ridiculous to have one meter for the entire building!