With a shower that age ,in a hard water area ,I would expect it to be scaled up to some degree ,and the new hose/ head have caused that bit more back pressure ,enough to trigger the prd. But for the price of a new prd its worth trying ,before thinking of a new shower. Good luck ,do let us know how you get on.
As Terry has more or less said you have probably fitted an unsuitable shower head which has created too much back pressure which has caused the PRV to operate.