I think your problem might be that Quebec is not a country. It is a part of Canada. Yet Quebec has some interesting features that make it feel more like its own country. The language is not even the same as in other areas of Canada.
To get a better answer, you might want to ask a more focused question.
The "problem" of nationalism in Quebec stems from the fact that Quebec got its beginnings as a French colony rather than an English one. In the Seven Years War, Great Britain took Quebec from France and incorporated it into what is now Canada. The people of Quebec have remained French-speaking and have more of a French culture than Anglophone Canada has. The two parts of Canada feel distinct from one another, which is why there is a nationalist movement in the province.
This can be seen as a problem because it tends to pull Canada apart. It tends to pit Francophones in Quebec against both Anglophones and Francophones in the rest of Canada. This nationalist sentiment is problematic because it calls into question what it means to be Canadian and it makes Canada less of a cohesive country.