I too suffer with what sounds like a very similar problem...I exercise 5-6 times a week varying weights and cardio and classes like circuits and swimming...but by Thursday/Friday I am exhausted...my diet has gone from really bad in October 2011and obese to really good by November 2011 and 30lbs lighter.....water intake is good but still I am exhausted! Any ideas?
I wonder what you are eating after your workout. It is great that you are sure to eat something within 30 min. after you're done. It is suggested that a post exercise meal should have a carb to protein ratio of 4:1, (4 grams of carbohydrates to one gram of protein) and low in fat and fiber, to increase the absorption of you food, and delivery to muscles. Also, the body is only capable of replacing 240-280 calories back to your tired muscles every hour. My suggestion (I'm not a physician but just a regular exerciser) would be to eat 40-50 grams of carbs with 10-15 grams of protein right after your workout and every hour after, for 4-6 hours and see how you feel.
I think you're probably dehydrated...watch your urine, it should be clear/almost clear. The yellow color means its more concentrated.