Low grade fever with chills might be because of various pathologies. Need to know a detailed history of your symptoms to further opine. The doctor who had advised you USG abdomen might have considered it as a screening to rule out various pathologies like liver abscess, gall bladder stones , bile duct stones, renal calculi or UTI etc. The attached image of USG is very difficult to interpret cause of the limitations. So my suggestion is to get yourself consulted physically first so that USG findings can be corelated. Feel free to ask for any further queries