It sounds like Chronic Fatigue syndrome to me, you have all the classic symptoms. You need to see a doctor and have a epstein barr blood test run and a sed rate as well.
In the meantime, get plenty of rest, drink lots of water, and eat fresh organic veggies and fruit.
It could be a number of things, your eating habit might be the problem as it is with most people in America. If you are eating fatty foods like McDonald's and not getting enough water that would defiantly make you feel sick/tired/weak.
I would recommend instead of fast food, stop by a jamba juice and drink one smoothie as a meal once a day it cost about the same as a meal at any fast food restraint and you will feel your energy start to rise, you can also take some vitamin b12 it will help produce red blood cells t that will bring more oxygen to your muscles and drink plenty of water, at least a gallon a day. Not only will this help with the energy problem but it could help shed off a few extra pounds.
Not saying you need to loss a few extra pounds, but who doesn't like to be fit and healthy.
Hope this helped.
Your friendly pharmacy tech
Mr_King_Brad