Sometimes i have random waves of sadness and i have no idea why i'm sad. is this normal?

This could be: due to repression of sad issues from the past. Sometimes, if we don't allow ourselves to mourn a loss fully, our unconscious tries to get our attention. When this occurs you might cry and have no idea why you are sad. It is a good idea to meet with a psychologist and see what the waves of sadness are about.

DEPRESSION: Get an evaluation from a psychiatrist to help identify what depression means to you and whatever else may be bothering you. Consider recommendations for medication if any are given. Get a referral for a reputable psychotherapist to speak with. Meds and therapy combined are most optimal; no just one or the other.