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Unfortunately we can't comment from lack of information as we can only guess widely. Keep in mind that without an independent access to information like a photo or video, we can only go by what and how much are you are telling us, and there is precious little to go by to help us narrow it down.
Please take a preferably clear and day lit picture and post in this thread. You can upload via the icon on the bar above your post, via the green Attach files button below your post or copy/paste if you upload a picture elsewhere. How To Post Photos
If in doubt, contact the vet clinic and see what they say. But please be aware that the receptionist has the same problem with triaging as we do unless you can supply some more helpful details as to whether the brown spot is bare skin, a discolouration on the fur; whether it looks like something stuck in the fur or on the skin, where on the body it is, how big it is etc.
Can you please get a clearer close up and another picture that shows better the location on the body? That is unfortunately about as fuzzy as picture can get and we can also not get an idea of the location.
Please check whether you have found one of the two nipples?
Here is a reference picture from our sexing guide:
Here is our sexing guide which teaches you how you can learn to sex guinea pigs reliably yourself. We most strongly recommend to double check the gender upon arrival. Mis-sexing is sadly rather common. Illustrated Sexing Guide