Can our brain ability can be deteriorated?

I was a 3.4 CGPA student when I was in my Bachelor degree. Recently I was resuming my Post graduate degree after a year working. I'm realizing that my ability to remember/memorize has been deteriorated significantly. i.e: 1. before this, I just need to read once (of anything) and I could remember that key points but rite now I need keep read and read and read but still won't be able to remember as much as before. 2. I realize that I could not remember for more than 15 minutes. Even though I totally understood the lecture but rite after that I found myself not able to communicate/express even the key points of the lecture 3. I realize that I keep forgetting a lot of major things. 4. Last but not least, I feel that my mental agility has been dropped significantly. I am not be able to think deep and critically like before. I would like to know if there is a possibility that I suffer any damages on my brain that cause this to happen? or is there any available medicine to regain it back? wow! I'm totally down rite now.. feel like I lost part of myself..