Could be a few things….. Is any oil leaving the side of the pump where it is backing up ? And does the level continue to increase or stay fairly constant ? It might be a stuck ball valve in that half of the pump, or a bit of muck somewhere, a kink in the pipe between pump and engine, or worst of all, the main bearing cup may have turned in the crankcase, thus blocking the feed hole. You ought to eliminate that possibility first…. Disconnect the pipe from that half of the pump, and push a piece of rubber tubing over the end that you can blow down. Maintaining air pressure with your mouth, very slowly turn the engine over, with the spark plugs removed, and at two or three times per revolution of the engine you should be able to blow air through the gland as the timed holes in the gland line up with the oil supply hole. If you can’t blow air through as described you have a full engine strip-down job on your hands, and realigning the main bearing cup is a specialist job.
FINGERS CROSSED !