What color is the blotch?
Is it raised?
Where does it appear and where does it move to?
How long before it disappears?
Does it return?
Thanks, XXXXX XXXXX await your reply.
Thank you for the extra added detail.
This is a very unusual symptom, being that it appears every 6 months and lasts 2 weeks and has an unknown origin. I strongly recommend that you have a dermatologist evaluate it when it is apparent. Perhaps he will culture it, or will most likely recognize its appearance and then be able to treat it.
I wish you well!