If this problem started right after you did the oil change then I would suspect a collapsed oil filter. If it's a fram filter I would bet money that's where the problem is. If the problem was there before the oil change it may be crud in the pan blocking the pick-up. When you shut it off the stuff settles into the bottom of the pan and allows the oil pressure to increase until it has run long enough to pull any loose crap back into the pick-up.