Why do girls act like this with me? Ahh?

So its seems with some girls,(Mostly in my class [Sophomore or below] every time i talk to them, to just so a little interest or conversation to be just a friend or friendly, they seem to get weirded out or idk nervous maybe. They just say I'm "weird" and just stare back at me the whole time. Help !?? Why do they do this? (I don't creepily go up to them out of the blue, just introduce myself and talk a little one day, then another day get to know them just a Little bit more, then the "weird" thing happens) With Juniors and Seniors they always seem more responsive and more open to talk so idk, i just don't get it. Help me because it bothers me a lot.