I experienced the same "energy high" when I started supplementing with Vitamin D. It died down some as my levels went up, I still have more energy overall, but not quite "high" feeling like it was. My levels were 21, so also very low. I definitely did not overdose, I got my levels checked recently and they were up to 50 (in 5 months). It would be VERY hard, in fact nearly impossible, to OD on Vitamin D with levels as low as yours/mine were. I took 5,000 at first, but take 6,000-8,000 every other day or every couple days now, no issues at all.