Bearing in mind that I am NOT a doctor, it sounds to me like what you have is called a subconjunctival hematoma. Basically, a tiny burst blood vessel. These are usually caused by some sort of injury to the eye, but can happen spontaineously, and it would be my guess that a real doctor knows a couple other reasons they could happen..........
And since I don't know if high blood pressure and/or it's medications can cause this, but your doctor will, I advise calling him as soon as possible and letting him know.
If you don't have a headache, if your vision is not affected, if you feel fine except for the freaking horror flick eyeball, I don't think it's anything to be terribly concerned about, but call your doctor to make absolutely sure. This is your eye were talking about here!
That will be 5 cents please!
Peace
Greenlydia