Brown stuff on shirt after picking up piggie

Question
I have two female guinea pigs at around 4 months old. Recently, I have noticed that whenever i pick up one of them and sit her on my knee, when i take her away there is a brownish mark left behind. Also I compared her bum to the other guinea pigs bum, and I noticed that she has a sort of yellowish snot looking thing on it. Its very small, but i'm worried because none of my other guinea pigs have ever done that.
Should I wipe it away with a cloth or see a vet?
Help as soon as possible thanks!

N.B. I just checked her bum and the yellowish thing has gone?.?.?.

Answer
There are two possibilities here. Sometimes after a sow has urinated there is a little residual yellowish stuff left as you describe. Sometimes when a sow is in heat they will also have that. Not all of them do so you can't use that as an indicator that she's in heat.

The brownish mark might just be a little bit of poo that has rubbed off. It doesn't sound to me like something for a vet.