Hi! Can you please specify how often you are weighing, whether you are always weighing at the same time in the feeding cycle/time of the day and whether your piggies are indoor or outdoors piggies. it would also help us a lot of you please added your country to your details, especially if you are enquiring about issues where climate and local wether can play a role.
By 4-6 months the fast growth phase is over and any further weight gain happens more in spurts. Guinea pigs reach their adult size arounf 12-15 months just before they reach adulthood but are generally heaviest at the peak of their lives at 2-4 years old.
The daily weight fluctuation can be as much as 40g between a full bladder and tummy and an empty one. Just a full bladder is 10g heavier than a just emptied one!
The onset of cold (UK) or very hot weather (Australia) with sudden major temperature humps can also influence the weight negatively. We have members from both countries and in fact all over the world.
With so many variables in play and insufficient information I can't unfortunately not help you any further right now.
Please take the time to read this guide here. It tells you everything you need to know about weight!
The Importance Of Weighing - Ideal Weight / Overweight / Underweight
Since we have members and enquiries from all over the world, we find it very helpful if you please added your country, state/province or UK county to your details so we can always tailor any advice to what is relevant and available where you are straight away. Click on your username on the top bar, then go to account details and then down to location. This makes it appear with every post you make and saves everybody time. Thank you!