The poster who said that it couldn't count against you is incorrect. Unpaid & late medical bills can & will be reflected on your credit report if they get turned over to collections.
Would you not pay your electric bill because you didn't receive a paper bill? I highly doubt it. A) Because you know you owe them money and B) Because if you don't pay, your electricity will get shut off. This is no different. You requested & used their services & were aware that there would be a bill. If you didn't get a bill, you should've called & asked for one.
It's not unfair, it's business. Most likely they had an issue with your insurance company (if you have it, that is), or had some type of audit that turned up this old bill.
In any event, it's petty to only pay part of it when you have no grounds to do so. So, if you are content with your credit taking a hit for $22.00, then feel free to do so. Also, if you use this doctor frequently, they will not see you until the bill is paid.
In the end, you owe the the money, period.