Actually, this has happened to me after I teach a particularly intense spinning class. My eyes get totally blood shot and I've had friends ask me if I'm okay. Yes, I'm fine.
Why does it happen ? I don't know the exact reason, but I've read several sources that say it's due to an increase in intraocular pressure. This also happens to people who lift heavy i.e. body builders.
When you workout anaerobically your blood pressure rises. This can cause the pressure in your eyes to rise too. As a result, some people (like me or you) might experience blood shot eyes after an intense workout.
If this happens to you on a regular basis, you really should check with your doctor. Have them check your blood pressure, just to be safe. I haven't had any problems, but yes, this does happen to me occasionally. I really does depend how hard I am pushing myself. And my eyes do clear up quickly.