What causes low vitamin D in the blood?

I had blood test that confirmed a level of 14 for vitamin D. Dr. Has prescribed vitamin d in 50 thousand units twice a week. I'm scared that this is causing or has cause me to develop an auto immune disease since I've read that low vitamin d can cause auto immune disorders. I have been having weird sensations in body like warm water sensations in legs and sometimes arms. Insomnia and anxiety. And non stop face and was pressure.I'm scared for lupus or multiple sclerosis. Can low vitamin d in blood cause these muscle or legs warm water dripping sensations? Why would I have low vitamin d I get enough sun I eat healthy in general. My Dr said its rare here in Arizona. I'm a 19 year old hispanics male if it matters. Please help