I was at 19ng/mL in September. Will retest my blood in a few weeks. Had your symptoms and more.
Low D and magnesium. You can buy D3 and magnesium and start supplementing. US range shows 30-100ng/mL as normal. Anything below 50ng/mL is deficient.
You need to take supplements daily. 5,000iu of D3 is more of a maintenance dose. Try to work up to 10,000iu daily, along with magnesium and vitamin K2 (if you don't take medications that would be an issue with K2).
Retest blood in 3 months.
The only blood test that can diagnose vitamin D deficiency is a 25-hydroxy-vitamin D (25 OH vitamin D). Unfortunately, some doctors are still ordering the wrong test, 1,25-dihydroxy-vitamin D.
Yes it can be caused due to the deficiency of Vitamin D.