I've just had this experience with my friend of over 35 years. She is done with me and I don't really understand why: nothing has happened , we hadn't had a row, I just turned down a night out as I was already going out that night with someone else (which she knew about beforehand). She obviously feels that I've not been making enough effort with her but had shown no signs of being unhappy with things.
I've just let it go, it's sad but I won't have someone thinking they can monopolize my very scant free time.