Why would my crush totally ignore me and turn his back to me when he sees me?

The day before this happened, we got along really well and I had a long, pleasant and cheerful conversation with him:)!...But all of a sudden today, he totally blanked me and acted like I didn't exist.

I don't understand his sudden cahnge of behavior towards me.
Why would my crush totally ignore me and turn his back to me when he sees me?
Post Opinion