What could be the possible reasons of a weak memory at 19?

I don’t know if this is normal or I have weak memory. I feel this many times.
Just few days back I studied some concepts and understood them very well. After 3–4 days now when I am studying something which applies them, I can’t recall those concepts. I understood them and mastered them completely, but now I don’t remember much about them. It may just feel normal, but I can notice something is not as it used to be,learning used to be say effortless to me and its like pain. Its not like I am bored, I forget what I studies interestingly.
• What could be the possible reasons of this?
• Has anyone felt this way? Though it may just feel normal to most of you but its not.
• What could I do about this? (Other than going to a doctor, I want to understand this and see if really is a problem of some kind, because doctors just need money and they would give medicines to any normal person who goes to them)