I dont think the one time high intake of selenium would lead to sudden hair loss, but rather what you have been doing in the recent past.
How much selenium did you take before going up to 1200ug?
I just started taking 600ug yesterday (up from 400ug) since I take 400mg Iodine now as well - like you :) My hair never fell out abnormally but has become less since starting iodine. Then again, maybe that was hair loss afterall - maybe many people who think they lose a normal amount of hair are actually losing too much due to Iodine deficiency and dont know it! What is normal anyways...