score:6
The truth is, if there is something, it could be anything, there's no one answer. There is some sort of wide perception that all-inclusives water down their drinks, it may or may not be true and it may or may not be wide-spread. Generally what I'd expect is just selling super-weak c**ktails, the same trick that many 'free drink for doing X' places use.
However, in Mexico and surrounding countries there does seem to be a serious problem with adulterated booze. Scarily there's been reports of methanol being added to drinks there and people getting sick or dying from it.
Is that what was happening to you? Short of a full analysis of the drink, nobody can answer that. As the comments above say, it's probably just mixed with a cheaper version of the same drink -- or perhaps it's entirely a cheaper version with the labels switched.
Alternatively, if the drinks are locally produced they may just be made differently. Different countries sometimes target different alcohol levels which would impact the taste as will certain local ingredients. I assume these are imported products, but maybe not. Even if they're supposed to be imported they may be selling locally made knock-offs -- this doesn't seem to be that much of a problem in Mexico apart from fake tequila.
But also notice that your perception of taste is altered by your surroundings, which could be a factor here. And once you've decided that the place is mixing in something your expectation will also influence what you taste. Senses are weird.
Finally surroundings and expectation also have an effect of the 'type of drunk' you get.
So, it could all be in your head, it be locally made booze with local differences, it could be knock-off booze, it could be cheaper booze, I hope it's not methanol.