At least you don't have to worry about being the worst mother in the world. Clearly, from some of the answers, that honor belongs to me...lol! I work long hours, often have to travel for business, and think nothing of leaving her at daycare for a couple of hours to do something that I can't do with a three year old to care for at the same time (and, yes, sometimes that something is a little bit of mommy time).
My daughter is usually the first one there and often the last to leave--her teachers give her so much extra attention that it doesn't bother me a bit.
Unless she has it stated in her policies that the price is limited to a certain number of hours (some places do), she has no business saying anything at all to you. Perhaps, because you've been a customer so long, she feels some sort of familiarity.
Being late is another matter. Any other business would lock the doors...but what is a care provider to do? HOWEVER, it happens (sometimes even when you leave the office EARLIER than usual...lol). As long as you don't make a habit of it, a courtesy call and paying a late fee is more than adequate.
This isn't the way I planned to raise my daughter. I wasn't planning on my husband walking out while I was pregnant. Luckily, I had a great job at a great company that doesn't bat an eye if I have to stay home with a sick kid or if I have to cancel a business trip. It also allows me to support my daughter...something her father had decided is optional.
If it means that I can spend two whole days just playing with my daughter, I'll happily let her spend a few extra hours at daycare so that I can clean, do laundry, and finish the grocery shopping. It is a familiar place with a familiar routine...something she likes. I get plenty of 3-4 day weekends to make up for the rest.
You aren't wrong, but it would be great to work this out with her if you want to continue to patronize her business. Don't justify or apologize...just offer to compensate her for her time and make it clear that her opinion on your parenting is neither needed or appreciate (in a nice way, of course).