Agree with Kevin and add
Did you just 'cleanse' the oil system with some sort of chem? It might have loosened something to partially clog the intake of the oil pump
Agree with others...did this happen after your oil change and filter change? Some filters, even the best, can has a marginal one to limit oil flow. If it has a high flow restriction, it will become a limiting component for oil pressure
Also agree with Kevin and your mechanic should likewise use a 'good' pressure gauge tapped into the line to check it.
Otherwise shooting in the dark changing out parts to just by chance nail the problem