Recently I've gone up quite a bit of weight in standing calf raises, on the machine I'm usually doing the whole stack of weights, or close, where not long ago I was only doing about half the stack.

I can lift the weight, but after three sets of them, I can hardly stand up. The middle of my foot hurts BAD, it feels almost like the bones in my feet are about to snap in half, although I'm sure that's not really the case.

What could be causing this? Tendonitis perhaps? I've been really accelerating on my calves, I hate to back off them now.