I agree with
@Resurgam
The manufacturer may have adjusted something in the formulation which is affecting you.
We have had members post to say that their pharmacy provided a different brand from their usual one, with some unpleasant effects.
In your situation you may find things settle down, but if they don't you could try asking for another brand and see if that suits you better. It isn't necessarily that change is bad, just that it may take some adjustment.
As for whether it is possible to develop a reaction to something after years of use, then yes, most definitely it is. That applies to makeup, deoderant, foods, fabric softener, medications... you name it, its possible.