I experienced a lot of this in October and November 08 and still do when I have bad days. I went and got a physical and my Dr. did bloodwork on me. Found out I have Hypoglycemia. I use to be a sugar-a-holic. Eating and drinking lots of sugar every day. It is a struggle for me not to have caffeine/sugar,especially sodas/tea. I have low blood sugar and have to limit myself on my sugar that I eat. I now have to have small meals and eat small portions of anything with sugar or avoid the sugar. My memory I noticed went first for the worse and started last summer. My memory is worse when I have too much sugar. I was very tired and knew something wasn't right when I couldn't remember anything. Definitely get it checked out by your Dr. Don't feel bad, I've looked for my keys and one time I was holding them in my hand while I had other things I was holding and covered my keys up!
Do you eat or drink a lot of sugar? Is there Diabetes in your family? Try eating less sugar/sweets and see if you feel any better. What you eat and drink has a huge impact on how you feel and how your body reacts. If it is Hypoglcemia, I have some books that I could recommend to you and a Dietician that helped me a lot that I can recommend. Try eating every 2-3 hours a snack/light meal, if it is a normal meal time. See how you feel after you make a few of these changes. Feel free to email me and I can talk to you more about Hypoglcemia.
I hope you get to feeling better, I've been there and it doesn't feel good to be tired and your memory fails you!