Vitamin B 12 deficiency.?

I have been facing problems like digestion problems, nausea, full body fever, irregularities in menstrual periods, heavy breathe, headache, difficult to remember things since 2-3 years. I have been on medication for digestion problems for last 1.5 years by homeopathy which I have stopped since last 2 months. Now I have heavy breathe, nausea, headache, restlessness, etc. I started going to Gym and there a dietician told me to get a test B12 test to clearly find out the problem. He also told to first detoxify the body by having meals in a span of 8 hours and liquid diet on other 16 hours. I haven't consulted about this with any doctor. I want to get Vit B12 and Vit D test. Kindly suggest me an appropriate way and when it should be done (during or after a week of detoxification)?