I would have to agree, you have a random spontaneous mutation, it is not common, but has happened before.
I had a friend (who is a biological scientist) breed a pr. of normal Argentine boas together and get a caramel albino in the litter of 12 babies. The parents were normal looking, and in future breedings produced a total of 54 babaies with only 1 caramel, so it was determined to be a random spontaneouse mutation that popped up.
On the plus side, you now have a truly new/fresh line of pastel that is unrelated to anything else on the market.
Congrats.