I've had the same situation as Seesaw, where a friend stopped inviting me to stuff abruptly, and upon later reflection I knew it was because I was being a downer. Sometimes you have to "read the room," so to speak. Or, in the case of friends, read the friend. Some friends are a good shoulder to lean on when you feel sad, some are just in your life to hang out with, nothing more, and they don't want to get involved in your emotional life that much. That was this friend for me. She just wanted to hang out and have fun.

On the other hand, sometimes people are just fickle. No other way to explain it. I agree with whomever said it above, unfriend her on FB and move on. No sense in trying to pursue something which is hurting you.