A ton of things can cause this. But if you keep track of where your gold is coming from seeing what it is should be easy. Mouse-over your gold and it'll tell you where all of it comes from. look for what changed between this turn and the last.
Possible reasons:
- governors switched off gold-producing tiles to other tiles in the cities. This can happen if you just came out of unhappiness as they will sense it is time to grow your cities instead of make money. - manually switch them back
- deals with the AI ended that were giving you gold - redo the deal
- a trade route has ended and you need to reassign it - send trades to new cities to get your gold back
Gold income can fluctuate a lot late game which is why you want it to be higher then +7 gpt. I recommend making markets, banks, and stock exchanges to improve your gold income and making and sending more trade routes if you haven't built all you can yet.