Why do I keep hallucinating!?

Before you say anything, I'm not on drugs. It started last Wednesday when I was walking home from school, I ducked because I thought I was about to walk into a sign but I looked back and there was nothing there. The other day i kept getting scared for no reason and kept seeing things that were not there. It's still happening a few minuted ago I thought my cat walked over to me but he wasn't there, that's what made me think about putting this question on here.