An amaryllis surprisingly bursts into flower

I brought the various amaryllis bulbs which are still in their pots into the house, thinking this was their dormant season and they would just die back. The others still have their long green leaves as well but this plant must have suddenly produced a new flowering stem when it was hidden behind a framed picture I'd moved recently.

When I picked up the painting, I saw that three of the flowers had just opened, probably because they had been in full sun inside the back window. As my movements are still quite restricted I couldn't easily move the pot so had to take this picture from the doorway to get far enough away to focus on it.