Is this girl playing me or am i her plan b.?

There is this girl that i like. We talk allot. She would always come up to me first and talk to me. She flirts with me allot. She looks at me allot. Everybody thought we where dating because we would get really close to each other. I really thought she liked me till the day i sow her sitting with another guy. I sow her look at me but she made it seem like she didn't see me so i just waived my hand to her to say hi and walked away. The thing is every time she sees me she always comes up to me and would talk to me for awhile but that time when she was with that guy its like she was trying to hide from me. When that guy left she quickly came up to me again and started to flirt with me. I first thought that they are friends because when he left she came to me and we talked for awhile. Second time i sow them again but they sat really close to each other and they talked for a long time and this time they both left and she didn't even say bye to me. I sow them both get in one car and then they left somewhere together. I've been really confused about this situation. Does it seem like i am her plan b. She makes me feel like she likes me but there is another guy and when she is with that guy i am like no body to her but when that guy is not around she is all over me.