What Causes Uncontrollable Anger While On Gabapentin?

I am concerned about the drug, Gabapentin, which I have been taking at various doses for over 5 years. My current dosage is; 800 mg tid. My number one concern is that I get angry quite often which has, at times, bordered on rage. I am VERY concerned about this change in mood because I have always been an easy going person by nature. I have had debilitating spine issues since 2010 with nerve pain to the S1 nerve root on the left side. I have had surgery on 3 different occasions to the L4, L5 & L5, S1; L4, L5 & L5, S1; L5, S1 respectively. I have been taking Vicodin since 2007, which I thought was the cause of my mood swing until I talked to a friend who is also taking Gabapentin; she is taking 3 different anti-depressants, whereas I am only taking one, that being; Nortriptyline 40 mg daily. I have serious depression and anxiety; at times I get very hyperactive, especially with an upcoming event such as a visit to/from my brother, if my brother can take me fishing, etc, and then I get extremely depressed the day after the event; almost as if it is a big let down.