The fast flashing just shows you that the indicator electrical
load on that side is much reduced - usually caused by a blown
bulb.

But a bad electrical connection can cause the same symptoms
(bulb not lighting properly, and relay switching fast).  This
can be either in the feed wire to the bulb, or in the
electrical earth path from the bulb body to the car chassis
proper.

If other lights in the same bulb holder group are also dull or
not lighting at all, then it's almost certainly the earth
connection that's faulty.  Maybe a bulb holder securing screw
is corroded.

Were the replaced bulbs actually blown?  Or did replacing them
just disturb something enough to temporarily remake a corroded
connection?

John