Hi, the pictures are not visible, can you try again? Guinea pigs commonly get trichoepitheliomas which are most commonly benign and the most common spot for them is near the tail/hind end. That being said, any mass on any animal has potential to be cancerous until proven otherwise. Your vet could aspirate some cells with a needle if the mass is big enough or simply remove it which is recommended while it is small. Even though trichoepitheliomas are usually benign, they can get very large and uncomfortable or ulcerated so removal while they are small (to prevent surgical complications) is recommended.
Contact the vet that did the biopsy and find out if it’s normal.