Mild sense of disorientation and light mental confusion after taking Vitamin D

I've always had a Vitamin D deficiency. Since I started trying to supplement it, I've experienced a mild sense of confusion twice, in two different periods, which coincides with taking Vitamin D.

Initially, my doctor prescribed a single dose of 50,000 IU of Vitamin D to take once a month. Now, I'm taking 2,000 IU per day. In both cases, two years apart, I noticed the same side effect.

Anyone experienced the same? Where should I start to investigate the issue?