500mg is hardly too high of a dosage. That's like the minimal dosage for adults, at least here in Germany.
Signs of overdose would be stuff like nosebleeding, bloody stools and possibly tinnitus.
I'm not sure how fatigue could be caused by aspirin. To my knowledge, the only study done on a link between aspirin and fatigue was done on multiple sclerosis patients and if I remember correctly, it was succesfully used to treat fatigue. Anecdotal evidence also suggests that aspirin has a mild stimulant effect. But again, I'm not sure about the biochemical cause for such an effect.