What?  Do you mean they changed your withholding amount and your paychecks are less?  What really got lowered---your actual salary or your take-home pay?

Wondering who your employer is, as that sounds like a piss-poor excuse to me. The tax law changes of 2018 actually caused folk's take home pay to go up, and up quite a bit for many. For them to use the excuse of taxes making your take home pay go down is a bunch of crap.  More than likely whoever told you that has no clue themselves, or else they actually reduced your gross salary. (Maybe time to change jobs?)

However, it is possible that your state taxes went up. I know in CA as soon as the TCJA (Tax Cuts & Job Act) was passed lowering federal taxes, CA immediately increased state taxes so the state could basically steal your savings from the federal side. No surprise for CA since they're doing everything possible to run that state's finances into the ground.

Well at the beginning of the year the new tax withholding tables come out.  So your withholding amounts may change. Ours always change.  Even on our pensions.

Also health plan premiums change .  You have to Compare your last 2018 paystub with the new 2019 and see what is different.