I would suggest if you are concerned, weigh your guinea pig at the same time daily for the next few days and look at the difference in weight. This will give you an idea about normal fluctuations in weight from day to day. Just like any living thing, including us, it depends on when you weigh them and their weight will naturally go slightly up and down,depending on how hydrated they are. 20g is not anything to be concerned about yet. It is only if the weight loss continues then this would raise a concern. You will more than likely weigh him again and he will have put the weight back on, but make sure you weigh him at the same time each day to get a more accurate reading. Just like us, we can weight slightly less first thing in a morning and more in the evening. At over 1300g already, he seems quite a healthy weight. Just keep a weight log for the next few days and you will then see by how much his weight varies from day to day. It will also give you a baseline from which to work, if in the future he does become unwell.
