I used to lack the motivation to do anything. I wouldn't eat, talk, sleep/awaken... It felt like I was being forced to exist. Honestly, I didn't want to. The feeling is coming again and I really don't want people worrying. Why was/is this happening?

Because you're letting it. This is a matter of attitude which could be a result of circumstances surrounding you.  You need to find something to keep your mind busy and off this and thus your attitude changes too and youll be better for it.  At least you're realizing what this is and you may need to seek professional help to overcome this but if it helps then its worth it.  Try and keep your mind occupied and let me know.  Good luck