it could be the change in the weather, spring makes me feel weary for the first week or so. How's your diet? If you do get a nap do you feel better? How is your skin and hair? Have you had any extra stress recently?
One other thing from recent personal experience. I have been feeling tired and sleepy recently...thought it was my usual spring reaction....then i got a sinusitis that i couldn't shift and I ended up getting an antibiotic course from the doc. Not only has it sorted the sinusitis, I feel generally very much better and more lively which makes me think that I have had an infection somewhere in my body for some weeks.