To be honest I am not sure what it could be, so I shall throw a couple theories out there and hopefully someone who may know more can confirm at least one of them. It also depends on the spells you have cast, as if they are spells over a week or so they can continue to "take" energy from you. Also if you tried creating a servitor recently they could be draining you of energy.
You could not have sent out the energy correctly and as such it is slowly being released instead of all at once.
Your spell could have drawn some kind of entity to you which could be draining you, or could just be the cause of the nervous feeling.
You could simply be inpatient, even on a subconscious level, and that is causing the butterflies and nervousness.
During the dream you could have cast the spell (it is possible, though I have experienced it rarely) which could be the cause of your tiredness and numbness.