There's a woman that has recently started to treat me poorly, and I can't figure out why. I would like help in designing the most efficient social experiment that helps me to solve this riddle.
If it's not clear from the disclaimer above, this post is about a personal situation and contains details about the two persons involved and their feelings.

Some possibly useful background info: it's about a year that we dance regularly together. I like her a lot and some months ago I have told her so, trying to be as level-headed as possible. She replied that she is still in love with the guy from her last relationship. We also talked about the fact that I'm polyamorous while she is strictly monogamous, so this situation could never evolve into a relationship. Aside from the occasional remainder that I like her and I find her attractive, there were no apparent ripercussions of those talkings: we kept on dancing together and being casual friends. In her own words, I've never made her feel uncomfortable.

However, it's two weeks that I sense that she is treating me with contempt, and I have no idea why. It seems to me that nothing in our relationship has changed, and this tells me that my model of the situation is way off-track.

There were two specific occasions that triggered my self-respect alarm: in the first one, while we were dancing she said something like "I tripped and almost fell on you. Oh but you would be happy if I accidentally fell on you, right?".
In the next occasion: while we were sorting for costumes provided by our dancing schools, I said "I'm going to need an XL size, the L size just doesn't fit" to which she interjected "Oh no, you're going to need a triple X size."

These are some possible explanations:

• she is not really treating me poorly. I'm being oversensitive and in her mind these were unrelevant remarks, or this is just some bizarre way of flirting or attracting my attention.
• She is treating me poorly, but there's no systemic reason behind: something totally unrelated put her in a bad mood on two occasions, and I just happened to be around.
• She is treating me poorly, there's a systemic reason behind, but it's not related to me: the systemic version of the one above. She is in a bad mood for a recurring cause, wether it's the tension for the incoming school performance or something else unrelated and for some reason I'm her preferred punching ball.
• She is treating me poorly, there's a systemic reason behind, and it's related to me: I've read that a woman with low self-esteem may start to despise a guy who likes her. The reasoning goes along this line: "I'm worthless and he likes me, so he likes worthless things, he must be worthless too."
• Something else totally alien: an unkown variable or a constrain of the system I've never thought about, an "unknown unknown".

I've layed the conundrum at your feet, let's see if you can suggest a way to unravel it.
If you need more information ask here or in private and I'll do my best to answer you.

ETA: changed "girl" to "woman".