I'm a girl, but this girl flirts with me all the tym for days then ignores me and starts flirting again another day && she has a boyfriend.

It means she is sexually confused. She only has a boyfriend to cover up her tendencies for girls she may be bi-sexual.

It could be that, or she might be joking around. Sometimes girls get a little friendly just because they think it's fun. She might not mean anything by it.

Does she ignore you when other people are around,and talk to you when they are not?Because I could be that she doesn't want someone else to see that she is talking to you,(like her boyfriend.)
Or if her boyfriend is there she might be to busy talking with him.