What are the signs that your kidneys are being rejected by your body?

Maybe none. The only sign early on be deteriorating function and therefore a rise in creatinine.
Signs are unreliable. The best way to diagnose rejection is through blood tests and ultimately by biopsy of the kidney. In this way, the rejection can be reversed before permanent damage to the kidney occurs. By the time symptoms develop, it may be difficult or impossible to reverse the process. These symptoms may include fever, pain and swelling over the kidney, and a decrease in the volume of urine produced.
They can vary. The signs and symptoms of rejection may be limited; but decreased urine output, swelling and worse high blood pressure; fever or chills; pain or swelling in the graft area, could be signs of rejection. You should immediately discuss this with your transplant team if these symptoms occur. Commonly, there are no signs and it is detected through abnormal laboratory tests.