Am i rebounding or what?

So i was on an all-summer flirting chain with a friend. During this time i sort of did something similar with another girl, who is two years younger then me. Not wanting to hurt her, i tried to use the words "bud" or "pal" whenever possible, and started calling her my surrogate little sister. Well i crashed and burned with the first one, and in my bad mood i tried to deal with it by focusing on the surrogate little sister. I was very flirty and friendly to her, and startedd getting texts about how she cant stop thinking of me, and how her hearts fluttering. The thing is, its not that i don't like her or anything. I'm just not sure if i'm rebounding, and if i am i don't want to end up hurting her. I'm 17 and she's 15, which seems weird to me for some reason. How can i either work my way out of this, or tell whether i'm just rebounding or not?