Wow.
Just tell her, "Sorry, I can't babysit anymore for $15 a day. If you need me to keep babysitting, I will have to charge the going rate."
Find out what the going rate is, before you tell her this, so that you feel comfortable naming a figure.
She has totally used you. You should not feel worried at all about telling her this. And you DEFINITELY should not worry about her getting mad. If she gets mad, she's a lunatic, and you need to drop her like a hot rock.
You have every right to ask for more money~!!!!!
Don't let this woman use you one more day. It is time to learn to speak up for yourself, like a grownup.
And DO NOT FEEL like you owe her ANY explanation. Just say the sentence exactly as I said it, with no explanation. If she has the gall to ask you for an explanation, you can just (be nice, instead of truthful) and say, "because I can't afford to work for that little any more."
Everyone on this site will support you. Tell her now.
By the way, when I occasionally babysit for a friend, she pays me $15 an hour. And she pays her nanny something close to that amount. I don't think you should babysit for less than $8/hour.