No, this happens in wines all the time. Sometimes we winemakers have to degas our wines before bottling because of it. It happens because co2 tends to stay in solution in cool temperatures, and even an airlock helps "hold" the co2 in the carboy. What I mean is, when fermentation is over, not enough co2 is being produced to bubble the airlock, so the co2 in there makes the wine gassy.
If the fermentation was finished, you have no worries. I assume you didn't use wine bottles, though. Wine bottles + carbonation = blown corks.