Hi
Lumps sitting loosely in the skin are generally harmless but we cannot tell you what any lump could be without hands-on access and if needed an aspiration (although the latter can be sometimes problematic). There are simply too many possibles that all pretty much the same.
Thankfully the overwhelming majority of lumps turn out to be benign and only need removing if they become a problem. Your aim is to know whether it is a lump that just needs monitoring or whether it is a lump that needs dealing with depending on its nature.
Any guessing games on here won't help you because they cannot give you the answer you crave and ultimately only serve to fan the flames of your anxiety if this turns into an one-upmanship showing-off game as to who can come up with the most obscure information.
It is one of the reasons why we have introduced section monitors and mods to handle the advice because the owner's mental well-being is as important to us as the health of their guinea pigs. In our view, a thread should firmly centre on both the piggy's and owner's needs and not on who knows the most and is the greatest 'expert'. None of us is a vet. Different people may mention different aspects or avenues but you will never get a quarrel over who is right unless something mentioned is plain wrong, dangerous or seriously outdated and useless. We try not to bring up any worst case scenarios until they really apply and then you'll get our full ongoing support.
We can give possible avenues to check yourself at home to help narrow down an issue wherever possible or to help you know what to bring up during a vet examination especially where vet access is not easy so a member can make the most of it by asking the right questions - but that is an entirely different kettle of fish. And that is why your location can play a role in what we are asking or mentioning so your piggy gets the most help possible wherever in the world with our nuanced and individualised approach on here.
All the best!