You do seem a little overwound jura if you don’t mind me saying so.
Sterling exchange rates fluctuate up and down always have. Swings & roundabouts! This means ex pat pensioners (like yourself as well as others) will, by default, go through boom and bust times as their pensions, paid in sterling, have to then be exchanged for euros.
This has always been the case.
Brexit, of itself, cannot be held responsible for those windfall (or hardship) months. Ex pats knew this when they chose to live abroad.
Of course the health insurance is another matter entirely as Brexit is responsible. That said, reciprocity has not been fully negotiated yet.
Spain (at present) is looking into this situation and so far ex pat pensioners on the Costas might well be pleased, as the Spanish government, acknowledging that retirees are not looking for work and are freely spending their hard earned disposable income out there in the bars, restaurants, hair salons, public transport etc. are a valuable source of income and are working to negotiate an amicable outcome.
It’s early days yet to get het up. Chill your beans.