My supervisor avoiding me -- what to do?

Professors are not always the people who would win a prize at interpersonal relationships.

Professors often have significant stressors in their lives.

So please don't assume this is about you. For example, perhaps the professor will be up for tenure next year and is trying to crank out as much work as humanly possible, despite feeling almost crippling anxiety, and has decided that it's necessary to minimize non-productive interactions so as to maximize the hours in the day available for research.

If you yourself are feeling anxious, unsure whether you are cutting the mustard, then ask. If it's difficult to ask this question in person, then you could send a preview of your question via email, a few days prior to your meeting, to give time to prepare. Here is one way such a question could be broached:

Dear Prof. X,
I am enjoying working hard on my project! But I would also like to find out how I am doing, objectively. Could we check in briefly about this when we meet next week? I am interested to know: What do you think is going well, what areas do you feel I need to improve in, do you have any suggestions for me, to get the most out of my time working with you, are there things you would rather I handle differently? This is the sort of feedback I'm hoping to get when we meet. I think that periodic constructive feedback will help me become an asset to your group.

Be prepared to take notes. Don't argue or defend yourself. Thank the professor for the feedback. Don't get too frustrated if your professor doesn't know how to do this sort of feedback session smoothly. Remember, the main purpose in asking this is so you won't have to rely entirely on your overactive imagination.

You should talk to him face-to-face when possible. I would say something like:

I might be overanalyzing this, but I noticed you've been a little distant lately. Have I done anything wrong?

If he says no, you need to leave it at that. He might just have a personal situation (e.g., parent dying), he might be concerned that he was acting too familiar and is now trying to reestablish boundaries, or it might just be nothing. I seriously doubt he would change your grades or fail to recommend you if he tells you flat-out that everything is fine.

It feels to me (without any evidence or proof, so I can certainly be wrong) like your supervisor might now regret establishing a relationship that's too personal on the spectrum between personal and professional, and is now trying to establish a relationship in a zone that he feels is more appropriate.

If this is the case, I don't think you have anything to worry about with respect to your grades.