why do some people introduce themselves to you and then when you run into...

[Queen Kitty]

...them they ignore you? or they give you the cold shoulder? why is that has anyone out there had that experience and do you think it's rude? can anyone out there share any experiences like this
Im not looking for rude answers please
I met a person that's my neighbor no biggie she just introduced herself to me and exchanged hellos and just small talk. Then I ran into her when she was with other people she ignores me. Another time I see her somewhere alone she says hi. The other people she was with I don't even know them and they don't know me either so I don't know if that chick just has some mental problem or is she just rude?
And it's been going on like that all the time she's alone says hello, she's with people ignores me. Then I see her at an event where she benefiting and she is warm and welcoming and acted like she never met me. Then I see her again with her friends ignores me Im confused.

Im also new to this neighborhood so that makes me feel more uncomfortable.
Thanks Simeen,
well I did say hello to this person on a couple of the occasion when I passed her by but she looked the other way. But youre right she could be going throught something.