Because your Vit D level is so low your provider is trying to build up your levels quickly. That is a recommended dose and is adequate, It is really quite difficult to get too much Vitamin D.
Vit D levels are low because either you are not getting enough sun or applying sun screen (which blocks the Vit D but protects you from skin cancer...) or where you live the UV rays are not strong enough.
Let me encourage you that this really does help with muscle aches and pain. You are on the right tract!
How could those be the only reasons vitamin D can be low? Are their any health conditions that I should be concerned about that are causing this?
I live in Georgia where the sun shines alot but have not been outside much because the heat is too much for me. I dont wear sunscreen at all.
Should there be any concern about adverse affects from the vitamin D?
You can also have trouble absorbing Vit D and that could be another reason. Most likely though it is due to you not being outside much.
Since an overdose of Vit D is so rare adverse side effects are not common or reported.