How do you know it was low vitamin D causing your symptoms?
I did X and now I feel better is completely meaningless without some quantitative supporting evidence providing a link between the two.
There’s a reason why the double blind trial is the gold standard of medical proof.
You may of course have this evidence, I’m just commenting on what’s written in this comment.
A billion people are vitamin D deficiency. Many are asymptomatic.
Did you have a viral illness causing your symptoms and it got better? Was it the vitamin D? It is hard to say. Some supporting evidence would be you had a previously normal vitamin D level back at your old GP.