Why do people do this!? This is aggravating?

I don't know why it irritates me so much but it does. There is a woman I work with, and whenever I say, "Thank you," she says, "Okay." What is wrong with saying, "You're welcome?" Or even and "Uh huh." ???? Why "Okay." It just sounds so rude to me to say okay to a thank you. It makes her look stingy. Can anyone inform me why some people do that?