I'm just going by what Cam Inman says here:
"Fleming indeed crumpled to the ground after moving at low-speed on a drill among the linebackers during Friday afternoon's session
. He spent about five minutes on the ground while being tended to be two trainers, defensive coordinator Vic Fangio and general manager Trent Baalke.
Fleming missed most of Friday morning's session after appearing to strain his left hamstring, which he said was "nothing serious" and that he'd return to the field as soon as trainers felt comfortable."
Two separate incidents. A strained hamstring wouldn't prevent a low-speed walkthrough.