PH dropping FAST

Hey, i have been growing hydro for a couple of years.

I use RO water and a bit of tap water to bring ph up.
It is a dripper system.

For the last couple of months, my VEG hydro water has kept dropping ph very fast(from 5,8 to 4,5 in like 2 days) the res. is 5 gallon. Prior to that, the ph has always been stable( last 2 years). The EC/ppm is stable, so it isn't because of a too strong nute mix.
And I change water every 2 months.

What could the issue be?

Is the dripper a new addition?
It screwed my ph.

2 months between res changes is most likely the problem.

I change every weekend.

Hey me too! We're on the same cycle . Actually as soon as my ph starts to drop I change. Sometimes it happens every 5 days when in mid flower.