What Causes Petechiae Skin Infection?

my age is 45, height 5.2, weight 170, fairly healthy, but experiencing lethargy the last year or so, which i try to take vitamins and vit D, as my last blood work said vit D was at 18, last blood work said it was 35, my question is, what would cause pitikia on skin, I have it on my arms, body, upper thighs red dots some are small, small a larger, circular and are filled with blood