FBI Director-designate William S. Sessions underwent tests as doctors tried to determine what caused internal bleeding that led to the loss of about two pints of blood over four days. Sessions, whose swearing-in ceremonies have been postponed twice because of abdominal problems, entered Methodist Hospital in San Antonio, Tex., after more bleeding from what doctors thought was an ulcer. Dr. Richard M. Rubio said the former federal judge underwent an abdominal CAT scan and was scheduled to undergo lower gastrointestinal tests today. Sessions, however, has not bled since he entered the hospital, making it difficult for physicians to pinpoint his problem, Rubio said. He said medication will control the previously undiagnosed ulcer, keeping Sessions out of pain.