Why did my imaginary friend come back?

It may well be because you are going through a transition and she is the only one that makes you feel secure so she's come back because you need the security and familiarity right now. Is she definitely imaginary? Like does she actually come from your imagination and you know that or is she a physical living being? I think that it's okay to have an imaginary friend... I admit it is odd at your age but as long as she doesn't affect your day to day life it should be fine as long as you realize that she isn't real though maybe you should consider seeing someone if you're confusing her with reality. Remember that you will make real friends at college. Best of luck!! :-)