I was 18 when i started to feel like you have mentioned im now 21. I still get
currently feel unmotivated, energyless, constantly tired and i really can't withhold anything in my memory for long
Im currently taking anti-depressants it doesnt stop any of the above, but everything else the feelings you have mentioned. It helps so much, it really was life changing in some ways starting them.
It good your aware of the changes because it means you can do something about them before they get worse. From what you have put it does sound like you have some degree of depression. Like Seraph has said
Although depression can be a chemical imbalance, in my experience there is often a trigger or circumstance that sets it off
events in your life can get it off, you have said you have big exams coming up which could be the cause or they could just be making it alot worse. Speaking to someone even if its just a friend you trust may make you feel better.
You should try and start to exercise again even if you go for a small walk with a friend because exercise makes a massive difference if you are suffering with depression, you wont feel like it or really want to go but do it for a week and you will feel the difference, you just feel alittle happier. Try not to shut your self away the more you close people of the worse it will become.
Yes drugs including weed can cause depression but this is normally from long term use and not from one or two. But its good you have stopped.
If you want to chat about anything feel free to comment below and ill reply.