2001 Dodge Truck oil pump/sending unit

I just had my heads on both sides replaced. Everything was fine whe I drove it home. The next day while driving to work my "check gauges" light came on and showed my temperature was fine but I had no oil pressure. When I got to work I talked to a co-worker and he said it was my my oil sending unit, which he replaced 3 times in his Dodge Ram. He also said I was getting oil pressure but the sending unit wasn't sending this info to my gauges. But when I was a couple mile from home that afternoon my engine began knocking like I didn't have any oil. The oil was fine so I thought it must be my oil pump. So, the next day I replaced it and cleaned out the trap. But, when I started my truck afterwards it was still knocking and the check gauges light was still on and was showing no oil pressure. please advise