Why do i get depressed for no reason?

i know everyone has their ups and downs, but like i really do get my ups and downs... its like when im happy im really happy and when im having a down day i get sooo sad for no reason and like ill be fine around people in general then when im by myself i get soo low and sometimes sit there for hours just crying and feeling suicidal (only on my bad bad days but happens often) when i know i have no reason to be... also lately iv been feeling really anxious, iv had panic attacks before, but sometimes i get a weird anxious build up for nothing... like lately iv been getting anxious that i wont be able to fall asleep and get really really anxious about it to the point where i just cnt relax at all and my heart is beating fast therefore not being able to sleep for ages, so makes me scared to try... i recently started smoking alot more weed (before used to do it bout once/twice a week for fun and chilling) now lately iv been using it as a way to relax and make myself sleepy so i dont get anxious before bed!! i dno whats actually up with me :/