Hi!
You need to step in feeding support asap. Please be aware that unlimited hay is making around 80% (the abrasive silica in it is also what is keeping the crucial back teeth ground down and the tough fibre is what the gut is wired to break down) while a limited amount of preferably leafy and green veg and 1 tablespoon of pellets max (please no more) are more in the way of a snack. So nibbling on a banana is more like nibbling on a small square of chocolate.
It is vital that your piggy gets plenty of hay based fibre. If she is not eating after treatment, then you need to step in with syringe feeding to make sure that she is getting the equivalent of ideally 90 ml in 24 hours into her in order to keep the weight up.
Please take the time to read these guides here; they tell you how you go about this and how you can improvise until you get any necessary support care in a pinch:
Complete Syringe Feeding Guide
Probiotics, Recovery Foods And Vitamin C: Overview With Product Links
Emergency, Crisis and Bridging Care until a Vet Appointment (including tips on how you can improvise with what you have handy or easily accessible)
Rather than banana, try and see whether your girl will accept dog pee free grass (high in silica, fibre and vitamin C), fresh herbs or very finely sliced lettuce. Grated carrot and or porridge oats may also help towards keeping the weight up but hay/grass based fibre is still the most crucial to keep the digestion balanced and it cannot cover for any necessary feeding support.
Unfortunately guinea pig dentals (or guinea pigs generally) are not really part of a general vet's curriculum so if they are inexpertly done, then you are sadly rather likely to end up with your piggy unable to eat after the procedure.