Hi and welcome
My Dylan had a dental root abscess that started out like feeling like a bony bump on the jaw.
If it is an abscess, it won't go away on its but will grow and become much softer to the touch. That is when it can be lanced, drained at the vets and stitched open (marsupialised) so it can be flushed twice daily with a mild antiseptic for as long as possible to allow the abscess to heal from the inside out but ensure that no little speck of infection is left inside to eventually start a new abscess.
Keep an eye on the front teeth - if they grow slanted, it means that the lump is causing pain and one-sided chewing and that the back teeth on the abscess are starting to overgrow. You have to brace for loss of appetite/weight loss. I would strongly recommend to monitor the weight very regularly by weighing on your kitchen scales first thing in the morning for best day to day comparison. Hay and fresh grass make over three quarters what a piggy eats in a day but they are generally the first food group that is impacted by any pain. The silica in the fibre is what actually grinds down the cavy back teeth while the front teeth are self-sharpening against each other.
Just watching a piggy nibble on a little veg can mean that a lot of weight can be lost quickly without you noticing. You have to also be aware that the poo output is running at least a day behind the food intake and that it is not an exact measurement. The kitchen scales are your greatest and most reliable ally in monitoring any health issue.
Make sure that you order some recovery formula so you can step in as soon as needed with support feed to make up what is lost in terms of hay intake. Your feeding support care can make all the difference.
These two very practical care links with lots of how-to tips tell you all you need to know and do if it turns out to be in an abscess indeed:
Weight - Monitoring and Management
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures