I let her just drop-in nominal amount something like $5 a week. Later she mentioned, that she works full time job(!), and moreover she enrolled to the local gym ( and they would not make any exception -you MUST PAY $$). THIS IS something that really hurts my feelings.
Quite a parallel here! You offered a reduced price to someone, yet they used you and lied. You were given a reduced amount, but how is this woman to know that you are being truthful? You have been shown that someone you trusted was not truthful. You were put in a similar position. Can you get yourself out of your own way, detach from the emotion, and see how these situations can be teaching situations for you? This directly relates to the woman at yoru studio changing her policy with you.
You stated that it hurts your feelings - this is a very personal reaction. Do you think she really was just trying to take advantage of YOU because of who you are, or do you think that she is just the kind of person who will take advantage where she can? In that case, it was not personal, and you should not be taking it personally. Myself, I would not have hurt feelings, I would be angry. Then I would have to look at myself and ask how can I avoid this in the future? That will take some real thinking and sorting out. You want to help others, but don’t want them to take advantage. Or, conversely, you want to help them but release yourself from the attachment of whether they are being true or not. But also, protect yourself. Be wary, but do not assume the worst of others. It is a delicate balance, and you do have to decide where you will stand in the case of being used by another. Sometimes we will knowingly allow a person to use us. We decide, do I want to allow this? And sometimes the answer is “no, I cannot abide it” but other times the answer is “yes, it is ok, it is causing no harm.”
I got the idea that she no longer goes to your classes but has joined a gym. If I am wrong, and she is still attending your classes, I would think that you should confront her in a friendly yet straightforward manner and inquire about her full time work, and then perhaps inform her that she must pay full price, as the special rate is no longer in effect. Or something like that.
You would have to think of the best way to say it. Plan ahead 
Edit: I have to apologize, I didn’t answer much of your first post, I did get distracted with the situation with the girl who lied to you, but I did see the parallel and felt that the two are more connected than you may think. 