I am of a low weight, and feeling very low in spirit and energy but all blood tests are normal except for a low vitamin d. i exercise regularly, ?

I think you are wise: to consider there may be something more going on here beyond vitamin D deficiency. Yes, "feeling low in spirit" can result from vitamin D deficiency, but I am worried about depression. Underweight can also be a symptom. Consider this and seek evaluation even if you don't feel "sad." Many have depression without recognizing depressed mood. Treatment is effective. Don't be embarrassed; it's common.