Found this which might help.
"Bumblebees form colonies, which are usually much less extensive than those of honey bees. This is due to a number of factors including the small physical size of the nest cavity, the responsibility of a single female for the initial construction and reproduction that happens within the nest, and the restriction of the colony to a single season (in most species). Mature bumblebee nests often hold fewer than 50 individuals. Bumblebees rarely preserve their nests through the winter. In temperate species, the last generation of summer includes a number of queens who overwinter separately in protected spots. The queens can live up to one year."
Apparently they are useful pollinators and like their cousins, the honeybee, are in decline, so I'd cherish your little colony tanith