you can oversleep as well. the body is an interesting thing, figure out how much your body needs and try to get into a regular routine. could also be nutrients based. try sticking to the recommended food servings [you might also wish to speak with your doctor to run a few test to see if it's nothing like low vitamin D, iron, or thyroid issues. i think diabetes can also make you lethargic but don't quote me] might also be a lack of physical activity causing your fatigue. still check with your doctor.
spiritually speaking however, you're probably spreading yourself too think and relying on your bodies energy instead of charging extra energy into your body to use in spells. you could also be forgetting to ground all this energy after your spell work causing a build-up which can also result in feeling weak. take a break and try to recover before starting again [of course, meditation and reading about magick should be fine, just don't try practising for a bit]