Why am I nauseous at work?

Hey, I'm 19 years old and physically fit. I was working 50hrs a week plus (stacking lumber) working side jobs on the weekends for the past 5 weeks. Last thursday, I had to leave work because I was almost vomiting. friday, i felt decent so i worked a little. today (saturday) i was helping a friend move some stuff, and when we slid a car dolly over (around 300-400lbs, but both of us were lifting), i instantly started feeling nauseous again. it's been around two hours since then and i'm still feeling nauseous. anybody know what's goin' on here?