Also, found this on the net:


"If you're talking about the coat, this is common as the weather warms. The dark color on a Siamese's coat (and skin) is temperature-related. Cool parts of the body are dark, warm parts are lighter. As the weather heats up, many cats shed their winter fur and grow in fur of a much lighter color, since the warmer outside temperatures effect how warm the skin stays. Then they darken during the winter.

This is true for the leathers, as well. I have a chocolate point whose leathers turn cinnamon pink during the warm months and darken in colder months.

Some antibiotics are also known to cause light skin/fur, and sometimes perhaps even spaying could be a cause, because of hormonal changes. It's also suspected that dark fur may lighten if there is not enough protein in the diet"

So maybe it could be after the op it was a cold spot so hair got darker or something to do with medication? I'm no expert though, I'm sure there are other people on here who can offer a better explanation!