If the spot is concentrated in one area I would probable agree with the cluster of blood vessles theory. The duration of the spot rules out a sub-conjunctival hemorrhage. A more diffuse area with deep redness could be epi-scleritis.
Did you have an injury to the eye? do you read or drive for more than 3 hours at a time? do you lift heavy weights? Are you healthy otherwise? How old are you? Have you ever had any eye surgery?
I'm 23, never any surgery or eye injuries. I do lift weights and use a computer/read a book quite a bit, but it seems that even when I don't use the computer/read, the spot remains. (I didn't do either of those much this summer, for example. Surely less than 3 hours.)
Do you think that lifting weights could lead to this kind of a condition?
Also very healthy, overall.
Yes, lifting weights can cause stress on the tiny threadlike blood vessles in the eye. Try to notice what you did within an 8 hour time frame before this occurs. Are you doing squats or just bench pressing? How much do you lift? Are you taking any vitamin supplements? Vitamin E can thin the blood. Sometimes you just can't explain why things happen, but if you break it down you might hit on it.
could be an inflamed pinguecula...
I do a whole body workout, including squats and bench press, but have only been doing squats for a few months now. I take protein powder as a supplement but that's about it. I also take a centrum silver at night before bed.