Apshouse could be right. Has the pressure been running low before today? If you change out the sender and still have a problem then get a mechanical gauge and check it after it warms up. Could be bearings especially the crankshaft bearings. Engine Oil Pressure Tester
Nothing is foolproof to a sufficiently talented fool.
I had this happen, it was the sensor, they're cheap you just have to be a bit acrobatic to get it replaced, out is easy, break off the top and use a ratchet, in is a bit tight you[ll need a flat wrench.