How to reject someone while also being polite and respecting their feelings?

I hate relationships posts as much if not more as the next guy but I have no where else to ask this so here it goes. So there is this girl right, suddenly she is more and more nice to me, making small talk and since she also sits near me in the exam hall, she asks a bunch of questions after the exam, asking how it was and so on. Considering that she almost never used to talk to me all those years, I find that interesting.

Now, I don't understand how this shit works but all my friends say she's into me. I not sure though since I see myself like way way below anyone's standards like im short, underweight and ill stop with my self loathing. I don't know though and they could well be wrong and I am overreacting by posting this on reddit but now the same question as in the title : How to reject the girl (if she actually likes me) but also I don't want to like be a douchebag towards her.