Is it in uneven speckles, various shapes and sizes?
How far does it spread?
Check the parameters as Kapo says, but bear in mind it could be a nasty bruise.
One of my 'Lotl took a leap and smacked his tail on the side once - there was an odd red splatter all across the fine membrane of his tail.
If it IS bruising, keep an eye on it, and it should clear up in a week or so.
I do have a picture if you request one.