This has happened to me on several occasions. It of course is caused by the rush of blood to the brain
Face/Off: On the return drop down(twice)
Vortex(PKI): While coming out of the 2nd loop(Once)
MF: Down the 1st drop(Once)
Mind Eraser(SWOA): On the Return Drop
It's a very normal sensation. You have probably experienced this before after sneezing really hard and repeadtely. It's not a life threating thing(Unlike what some doctors say)
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God Bless Coasters!!
http://www.jccn.org/religion/grace/Default.htm