Wow - Yellow Begonia Flower?

I potted this Begonia as a seedling last weekend. I had brought the seedlings about 4 weeks earlier from a visit to Townsville and every time I watered them I promised I would get around to potting them up soon.
I have been very pleased with how they were surviving after their slow start. Imagine my surprise today when I spotted the beautiful yellow flower on this one - I didn't even know that they flowered in that colour or had a flower that big, it's nearly bigger than the small plant at the moment.
As you can tell I'm just an amateur gardener but I enjoy it and love the surprises like I got today.