Lack of Vitamin D making me weak?

Almost 2 years ago I decided to lose ten pounds ahead of my sisters wedding (which I did, yay). However, while yes, I was working out almost everyday and eating right, I was also aching all over, my muscles felt heavy and tired quicker than they used to and I began to feel no interest in anything.

My arms ached when I washed my hair, my hamstrings and glutes felt week when I bent over the sink to brush my teeth. I didn't like to stand for long.

It was coming out of a bad winter where I had not really been out in the fresh air much at all, owing to work and the weather and I was pretty miserable.

I went to the doctor who told me it was stress and the exercise causing me to ache.

I tried eating right, but not working out, but I didn't feel any better. I went back to the docs worried it was something serious but she referred me for counselling!

I got used to how I was feeling but over the summer I noticed I felt much better. I went on holiday to the sun and was nervous about doing long hikes etc but I felt almost fine.

Come winter again and i felt weak, my muscles ached, I was tired..... I went back to the doctor who referred me to a neurologist which terified me. However everything was declared fine and he told me to basically go away!

We had a really good summer, I spent a lot of time outside, and while my muscles got tired with prolonged use - pruning a hedge made my arms ache for 2 days! - I felt better in myself again.

Now, it's winter again and it's all back. I go to the gym and it's so much more effort. I'm afraid of not keeping up in excerise classes sometimes my muscles just ache so much. I don't feel strong. I avoid doing stuff and have put on weight again.

I went to another doctor who tested my Vitamin D. She said I had some proximal muscle weakness which can be caused by thyroid or vit D deficiency. My thyroid and everything else came back fine but vit D was 'a bit low'. She said it classed as an 'insufficiency' rather than a serious deficiency and has put me on 800iu of Vitamin D twice a day for 3 months. I've been on them for one month and don't feel any difference what so ever.

I recently found some over the counter Vit D that's 4000iu daily. I think I might try that.

Has anyone else had this?