Hi and welcome
Please see a vet. Checking the mouth and especially the back teeth for something stuck in the between two teeth is a two people job, as is removing anything gently and safely.
The fact that your piggy is unable to chew a softer veg like celery properly points to a problem. Since the much tougher hay fibre makes over three quarters of what a guinea pig should actually eat in a day is hay, the inability to chew can quickly result in a much more dramatic weight loss than you would expect.
Not eating enough/chewing properly can then result in dental overgrowth and digestive problems as a knock-on effect if the problem is not checked over a longer time. It is the very abrasive silica in grass/hay fibre in which the dental growth rate in guinea pigs has evolved; the gut microbiome and the whole digestive process are also laid out for processing grass/hay and any supplementary wild forage, which we now replace with preferably leafy green veg, pellets and any treats all together lumped under 'supplementary'.
Please switch from weighing once weekly on your normal kitchen scales to weighing daily first thing in the morning when the daily weight swing is at its lowest for best day-to-day comparison. Step in with offering additional feeding support once 50g weight loss has been passed.
But please check first whether your piggy is still a good weight for their size or whether they are already underweight, as that impacts on the urgency of seeing a vet and you stepping in with feeding support - being able to feel single ribs means that there has already been some noticeable weight loss and your piggy is underweight. Ideally you should just feel the ribcage, especially as you are mentioning that you have noticed signs of discomfort in the mouth a while before. You can find advice on how to check for the 'heft' or BMI in chapter 3 of our weight monitoring guide link below.
Please take the time to read these very practical guides with all the how-to advice on how to go about it:
Weight - Monitoring and Management
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures
Long Term Balanced General And Special Needs Guinea Pig Diets
A guide to vets fees, insurance and payment support.
Emergency and Crisis Care as well as Bridging Care until a Vet Appointment
PS: I have added the emergency/bridging care link in view of this being a weekend.
All the best.