Could I Have Post-Botox Side Effects Due to Taking Gabapentin?

You are describing eyebrow lowering, numbness, and headache after Botox injection. I do not know of any known interaction with gabapentin (although I suppose anything is possible). One important detail that you left out is how soon after your treatment you noticed these symptoms. Headache is very common for a few hours after ijection - in fact, it is described in about 25% of patients after their first treatment.  Eyebrow lowering may be due to improper injection technique or placement of the Botox.  Numbness is unusual but may be a result of injection into a nerve.  Regardless of the reason, you should contact your injector and discuss these directly with him or her.

Numbness is not common to either botox or gabapentin, so you if you have sudden onset of numbness, that needs to be looked at as soon as possible by your regular doctor or a neurologist.

There is no interaction between gabapentin and botox.

Lowering of the brow (brow ptosis) is definitely possible if the botox is injected too low on the forehead.  The forehead muscles act to elevate the brow.  If they are injected less than a certain distance away from the brow, they will be relaxed and the lifting function will be less.

Headaches can often come from trying to elevate a heavy, descended brow. 

Please see your doctor as soon as possible about the numbness.