Angiomyolipomas of kidneys are known to be associated with an entity called Tuberous sclerosis
which can manifest with skin lesions. These are usually seen as crops on face and the angiomyolipomas also are seen in both the kidneys in such patients.
Without actually having had a look at the lesion on your back it will be difficult to predict what it could be, but it does not sound like a tuberous sclerosis lesion at present. Your being an RN makes it easy for you to understand medical literature, and thus if at next USG, your doctor picks up an enlargement of the angiolipoma or if there are fresh ones, you should be concerned and discus with your urologist
as to the next step. If there is no growth of the lesion, and it is less than 2cm, i suggest you can wait and watch if you have no symptoms.