oh, i know what's going on.
depending on the gender of your friend, and depending on your gender, there are 5 possible scenarios that I thought of:
1: He/She likes you, a male/female.
2. He/She likes you, a male/female, but they're not gay.
3. He/She got a girl/boyfriend
4. They've got a massive secret that they're worried you'll tell everyone about if you learn it.
5. Worried that you don't like them anymore so they're trying to get your attention.