Temporary blindness while standing up?

Im a normal 15 yr old teenager, i weigh around 140 (pretty skinny but no too much) and about 5"11, i drink a lot of water often and eat my meals regularly. when i watch t.v. i tend to lay down and enjoy the program but when i stand up i have a temporary blindness. I've read other questions about it and got similar answers (low blood pressure, not enough water etc) but i want to check if there are any specific suggestions for me or different possibilities.