The console would have shown something if there was a serious problem with the API key or backend setup. So, your problem seems to client related.
There are two possible scenarios I can think of.
Scenario 1. Your key was compromised and someone else is using up your quota. They would have to be a bit of an evil genius if they are systematically using up, say, 48 of your 50 requests, but not all of them. Seems a bit unlikely, but can’t be ruled out.
Scenario 2. Your browser is causing some problem, maybe an unusual plugin or browser extension. Simple to test out, just try a different browser.
Hope that helps.