Nucking, I would write (real letter?) your niece and tell her you noticed you're not on her friends list anymore and ask her why. Just one-sided wondering if you did something when you were manic isn't going to get you information or allow you to understand/see if you can fix the misunderstanding/problem. Does she even know you are ill? Do you know what your sibling has told her about your condition? You may just have been party to a bad comment from someone else and she may be embarrassed/anxious about how to respond to you so just "not" doing so.