Plank Challenge: I'm getting worse

I stayed on track with the plank challenge.

Thirty seconds? Please! Even the Monkey did that one with me (though his plank was more of a downward facing dog than a true plank).

One minute? Challenging! But, doable.

One and a half? Eh, that one was no fun.

Two minutes? I thought I was dying, but I got though it.

Two and a half minutes? I break out in a cold sweat five seconds in. By fifteen seconds I am shaking. By twenty seconds, my lower back is screaming. The last two days I've been trying to do two and a half minutes, and both times I only made it to forty-five seconds. How is that possible?

I am probably weak from lack of junk food.