Thank you for posting your question here, I will try to answer it to the best of my abilities.
The most common causes for protein in your urine are diabetes
. I am assuming your doctors have already concluded that you have neither of those two.
The next thing your doctors should check for is glomerulonephritis
, a biopsy
or CT of your kidney should help in confirming this.
So ask your doctors to check if this is indeed the cause.
I hope this helped.