Am I depressed

This is probably a dumb question because if I was depressed I would probably know but I just feel like something's wrong with me. I have zero motivation to do anything and have started to avoid friends and just stay at home and do nothing. My grades have gotten significantly worse. I have suicidal thoughts although I know I would never actually follow through. I have felt like this off and on for months at a time for the past few years and I don't know why. Sometimes it seems to get better just for me to feel empty again. I feel like I'm stuck this way and nothing will fix me.