If you used to get along with a student and then suddenly they act like they don't like you, my first thoughts are that the student has emotions related to something else that are being projected on to you. Let it go. Most likely the student isn't aware of how they are acting towards you and you are seeing it as a bigger deal than it really is. However, if their changed demeanor continues, maybe your relationship has been compromised somehow. Try to rebuild the bond by asking questions about common interests the same way you would with students you are just getting to know.