What is the reason behind the forgetting any thing after a few days?//

So many causes, but: In a 20 year old? Most likely stress and a lack of sleep. High cortisol levels have been shown to interfere with memory formation. Get 8 hours of sleep, exercise, and have a proactive study schedule to avoid all nighters and minimize stress.

Not much to go on...: Memory is a very big topic. Basic memory skills vary drastically among people. In a person they can vary through life due to all sorts of internal & external causes. In other words - this may have easy to find causes or not. It may be a brief thing or not. If you are worried, be evaluated by a neuropsychologist. Good luck!