An acquaintance of yours asks why you act so strangely around her. She seems a little hurt, and is worried you hate her or something. And the truth of the matter is that you have… well it wouldn’t be right to call it a crush, but you find her very beautiful and have been worried about making a fool of yourself. And it isn’t your intention to harass her or pursue her in any way, you’re not even looking for a relationship or anything right now, you’re just nervous around her for basically no reason, and she’s noticed and she wants to know if she’s done something to offend you.
What on earth is the correct thing to say?
Most people find honesty disarming. If you say “I’m socially awkward and I feel nervous around you. It’s got nothing to do with you as a person. It’s an issue of mine and I’m trying to work on it. I’m sorry if I’ve made you feel uncomfortable or otherwise upset you” she will probably accept that. And this is something you should be working on, BTW, although only you can really say what form that should take.
As some others have said, do not mention her looks or your crush.