I think most people have no idea where Bulgaria is, which allows them to make just about any joke about the country or its people. It's not that Bulgaria has become extra popular these days, it's just some unheard-of-country one could pull a joke about. And, because it's small and far off, the chance of offending someone in the States is close to non-existent. I also think your sister may have noticed the mention, because she is personally involved by you being there.

When I was in the States, someone asked me if we had TV back there or if I meant Bolivia instead of Bulgaria. Personally, I don't get offended by questions like that. You can't possibly know everything, so it's fine by me if you ask  me a question, even if it may seem ridiculous to me. What I get offended by is if one asks me in a bad way, like if they're being condescending to me.

Since I moved to Germany, I noticed that most Westerners think of the Balkan region as a big piece of... something. Most people think it's a cluster of countries that are pretty much alike, which, given the history of the region and specifically the Balkan Wars, is not true. The difference in attitude here, in Germany, is that most people have heard of Bulgaria or the other countries of the region, but they consider them to be all the same in terms of language, politics, economics and so on.