Take your guinea pig to a qualified small mammal (pocket pet) vet. There is no way to know from a description of 'lump' to determine what it is and whether it will kill your pet or not. The very fact it is growing is a very good indication it is harmful to your pet.

If you can't afford to take proper care of your pet, which includes vet visits, my suggestion is to give it up at a rescue or Humane society who is willing to ensure your guinea pig gets the proper care and treatment.
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PHLdyPayne