Why does it happen like this?:

One minute I'm fine, getting to spend some time doing what I want, rather than running eight million errands or doing work for classes I just don't care about. The next minute, I'm sitting in physics, feeling really overwhelmed, realising I basically have no idea what is going on or why I'm taking any of my classes, feeling ridiculously sick, and generally wanting more than anything else to go home and cry for an hour. I try to go to recitation, thinking it might start to fix the problem, but it's full. I'll try again in an hour, but at least now I'm remembering why I'm not going to grad school. I'm not sure how I'm going to handle another quarter of this, much less five more years. I love math, but somehow the learning of it in this environment at least doesn't do it for me.