Yea i'd be upset about that, I'd hate to think someone thinks poorly of me, given it's unjustified I'd say it's just her being petty and silly. I'm just currently being a preceptor to a nursing student for the first time, one year after starting work, and I love it! I can't believe how much knowledge I have gained, gee I remember being a student two years ago and wishing I knew as much as my preceptor knew then, and I am so proud of myself that I now know it and can teach it!
I am from a unit with very little turnover, however recently a lot of nurses are retiring, and so there are a few of us young ones coming through, and it's really nice to be accepted into a unit that is so close knit. If your unit is just being petty and toxic, hopefully you can find somewhere else to go that appreciates you more. Also, good luck with grad school! You never know, sometimes you have to take a step away from the floor to be appreciated.. maybe she'll notice how good you are when you're gone!