I dog sit as a secondary source of income, and have for about a year and a half now. While most of the people I sit for are strangers, I've watched a fair amount of coworker's/friend's dogs as well, and they have always offered to pay me for these services, even when it's only for a night or two. 

This particular friend, we'll call her E, asks me to watch her dog somewhat frequently. I love her dog, and don't mind watching him too much, but I obviously still have to go out of my way to make sure I come home often enough to walk him, clean up after his messes (he has frequent accidents), etc. She has never so much as offered to pay me for watching him, apart from buying me a latte here or there as a thank you. From the very beginning--and we became friends after I had started dog sitting, so she knew that this is a service I typically charge for--she has just expected that I'll watch him for free. When it's just for a weekend or something, it's not the biggest deal. But she has, on many occasions, asked me to watch him for 5-10 days at a time, completely for free.

I agreed to watch him for her the last week in January, but the longer this goes on, the more pissed off I get. I know we're friends, but I often feel like she takes advantage of me in this way, and I'm tired of it. Not to mention, when I'm watching her dog, I'm potentially losing money from other clients because I can only watch one dog at a time. 

AITA if I ask her to start paying me for this? I know I already agreed to January, so maybe I should suck it up and do that for free but then ask that in the future she pay me for it? I feel bad asking a friend for money, but I feel like she's using me past the point of a few friendly favors. 