Ah yes! Well, all bets are off when it comes to islands not terribly far from the mainland. (Assuming you do not mind my calling South America the "mainland"!)
You seem to be some 500 km from Venezuela, and less than 200 km from other islands, such as the Grenadines.
Now, at the same time you are very far off any major butterfly migration routes that I know of (bearing in mind that I know of very few!) But Barbados is nowhere near any position that any insect could plausibly have a migration route. In particular, Barbados is far out on the ocean, a watery grave for any but the luckiest insect. Such situations are not any good for migrants.
However, an insect does not have to be a migrant to be blown several hundred kilometres over the ocean, and land fortuitously on an island. That is how most islands got most of their for now and flora anyway: accidental travellers on wind, currents and birds. Of course most such passengers eventually died without issue, because not many of them brought mates or fertile young along. Although successful immigrants sometimes start major new colonies, the bulk of the emigrants from the land of origin counted as losses for the population. For a mainland this is not much of a problem but for small islands it is so severe that loss of flying capability is a frequent adaptation on islands. It reduces the loss of breeding populations as sacrifices to winds that blow flying birds, seeds and insects out to sea to feed the fish.
Now, that butterfly of yours.
Firstly, as I said before, there is a strong possibility that it is locally present, but that you happen not to have seen or noticed any of them before.
Secondly it could be an actual, deliberate or accidental, human import. There might for example have been a pupa, or even some eggs or caterpillars, on someone's luggage or a crate of imported goods. It even might have been an adult butterfly that sheltered on a ship or in an aircraft. This sort of thing happens all the time. Whether the species then becomes established or not is another matter. It might or it might not.
Thirdly, a butterfly might have been alarmed or caught in a storm and blown out to sea, without any need for an actual migrant route or population.
To get any less speculative assessment, I am afraid that you would need to supply either a specimen or a sufficiently good photograph for a local expert to give an opinion.
Good luck,
Jon
