My mom called me a slut?

No, you cannot be a slut for the number of boyfriends you may have or have had, what makes a person a "slut" is when they have indiscriminate sex with anyone and everyone they can. It was a very poor choice of words on your mother's part. But I think what he was trying to do, was to voice her feelings of losing you to going away to college, and her worry about your safety and well being-- you are her little girl, going off into the big bad world, and it scares her. Men are undoubtedly high on her list of possible threats to your safety and well being, so she grasped at what she could to put flesh to her warning of caution. I'm sure she does not think you are a slut, or will become one, but saying that you need to take care not to give that impression, as trouble could come from the appearance of being so. Good luck to you in college!