Hi!
Please weigh any piggy once weekly as part of their life-long body onceovery health check and have them seen by a vet if the weight drops for whatever reason. Your girl is currently at the peak of her life and should not be losing any. 'Mystery weight loss' is my least favourite symptom because there can be so many different things hiding behind it. It really needs a thorough hands-on examination.
There is no working weight chart for guinea pigs because sizes vary so much - an adult piggy can be anything between between 700g and 1800g and still the correct weight for the size. However, there is a way how you can work out whether your piggy is a good weight for its size at any age by feeling for the 'heft' around the ribcage. Please take the time to read this very helpful link here; it talles you how weight changes through life and how you work out the heft:
Weight - Monitoring and Management
Please make sure that hay is the mainstay of your diet (over 80% of the daily food intake) for long term health and a longer life. Veg and fresh herbs (max. 50g) and pellets (1 tablespoon per day) should be rather seen as a daily treat. while fruit and treats really should only be occasional special treats and not part of a regular diet. Sadly many owners mean well and inadvertedly mean too well instead...
Long Term Balanced General And Special Needs Guinea Pig Diets
Rather than treats, aim to make normal feeding time fun and enrichment by having your piggies work for their food - that is much more fun and much healthier!
Enrichment Ideas for Guinea Pigs