Might have been working too hard?

I am 17 and am doing the hardest job that i have ever done. Every week a hay farmer has me help unload 300 bales a day. This morning before i left, i was feeling like i just got out of a hot bath, but i still went. After about 200 bales i was feeling tired, but i kept pushing myself to keep going until it was done. When I got home I was feeling really crummy, and took a nap. Now I feel like I have to flu, but without nausea. I cant stand for more than a few minutes. What might this be?