That's odd--I get the exact same thing when I do calf raises in my powercage using teh safety squat bar (I just unrack the bar, use my hands to balance myself in the rack, and raise my heels up and down on a block). It happens EVERY time I do them, and only with that exercise. I have no clue what causes it but I'm glad someone else has it, I was starting to get worried.
And I don't even use a lot of weight for those, usually just put two plates on each side (which is 245 on a SS bar). I get a little pump in my calves but not a lot.
Anyone else?