Rihanna -- I've Got a REAL PROBLEM with Poland

Here's Rihanna in a tiny bikini at a beach in Sopot, Poland this weekend ... where a giant pack of Poles lined up behind a fence to gawk.

Rihanna wasn't thrilled with the attention -- actually, she was pretty pissed ... tweeting, "Tried to have a beach day, that was a fail!!"

RiRi later griped that she felt "caged in like a f**king animal!"

Fun Fact: Sopot is a popular spa town for the rich and famous ... and sports the longest wooden pier in Europe.