Sounds like a reaction from your medication. Either that or possible narcolepsy.

I have epilepsy, and after either a grand mal seizure (the real violent seizures) or an absent seizure, I am really tired, basically because, after any kind of seizure, the person's brain wave activity is irregular, and thus, it takes a lot out of your brain exertion wise, and makes you really tired. Thus, usually people after they have seizures need to sleep twice as long, so that the brain can relax and return itself to normal brain activity.

Many medications nowadays, even prescription allergy meds, have side effects that include exhaustion, sleepiness, etc. and that's why, usually, on the prescription bottle, it says not to operate heavy machinery within a specific time span after taking that specific medication, due to the side effect(s).

Anyways, rambling aside, I hope you heard back, or will hear back from your doctor soon about this, and get it checked out. Best of luck.