So I was cleaning my guinea pig's cage and saw what looked like blood. It might have been juice from something that my parents gave her but it was in the corner she uses the bathroom in. I have a female guinea pig who is 3 years old. Do guinea pigs go through menstration cycles? I haven't really noticed it until now (who looks at their pets poop?). I'm just curious. Thanks!

Guinea pigs do not bleed during their heat cycle.

You mentioned poop. If there is blood in the feces it needs to be checked out by a vet.

I do have to ask, why are your parents giving the guinea pig juice?

Since you found it in the "bathroom" area, it is possible that it could be blood in the stool or urine. Either way it needs to be checked out by a vet.
Actually, looking at your pet's poop is something we should all do, it can be a very important indicator of their overall health and alert us to problems they may be having.
What would your parents have fed that would look like blood?

Here is a site that is recommended by the ASPCA, lots of good info in there
http://www.guinealynx.info/healthycavy.html