Obviously, I'm just a nobody on the internet...
Vitamin D is a fat soluble vitamin. You can't just start taking it and expect it to 'work its magic' because it takes time for your blood levels to reflect supplementation. It's not like popping some vitamin C or B vitamins which are immediately available for your body to use.
The ONLY way to know your vitamin D status is with a blood test. A doctor can do it, or you can order a test kit for home (you have to be able to prick your own finger for the home kit).
Also, if you're taking supplements, make sure to take it with a meal containing fat and ideally in an oil suspension, too. You're probably totally wasting your money if you take it on an empty tummy. Fat soluble vitamins need fat.