Possibly two different coloured plants in the pot?

A photo would be very useful - there is another possible explanation for the change in flower colour. Tthe plant commonly known as Christmas cactus is Schlumbergia, and originally, only came in pink. The plant known as Easter cactus, which looks similar but is a different plant, had red flowers around spring time. In recent years, a lot of breeding has been done by growers, so now there are red, white and pink varieties of Schlumbergia - but sometimes, with more recently created genetic crosses, the DNA doesn't hold, so to speak, and this might mean a plant you bought when it was red may, the following year, have a genetic blip and produce flowers of a different colour.

Bamboo these always used to be called Zygocactus truncatus - have they been under Schumbergia for a long time?

Not sure - its called Schlumbergera truncata now - the one's that's changed its name completely is Rhipsalidopsis gaertneri, also known as Schlumbergera gaertneri, the Easter Cactus - its Latin name is now Hatiora gaertneri. Gets more and more difficult to keep up...

Oh crumbs. We used to have Zygocactus at Christmas and Schlumbergera at Easter. Never heard of Hatiora. Think I'll stick to English names...

Blame the geneticists - having cracked the human genome, some bright spark then started going through Flora to check genetic origins, hence the huge number of name changes and reclassifications.

Hmmm...