What does it mean when a rabbit's urine is red?

It is very unlikely that there is something wrong with your rabbit. Actually, buns pee orange/red for little reason. It could be because of the winter, something in the food or, at times, it's just a color. Even when you feed two buns the same food, one might pee orange while the other one doesn't. More information is available on http://www.blurtit.com/q850680.html
Rabbits' urine varies in color from clear to yellow to brown to bright red. This is usually not a cause for alarm unless there are additional signs, such as sitting and straining to urinate, loss of appetite or a temperature. When you see red urine, don't panic; just keep your eyes open for other signs that may indicate a problem.

The red color will usually be gone in a day or two but can last for a much longer time. Actual blood in the urine would look like urine with red specks. If you're in doubt, don't risk your bunny's health -- have your vet test for blood in the urine.

You can have 20 different rabbits in your barn and feed them all the same things. One rabbits pee could be a creamy yellow and the others a bright orange. It is just the different pigmentation factors in each individual animals kidney's.