Does my guppy have an internal parasite? The chest cavity of my guppies have a lump. Definitely not dropsy?

Your guppies have dropsy or TB. Both illnesses can make lumps that eventually turn into open sores. I know you say it's not dropsy, but look over the top of your fish tank and make sure their scales are not poking out like a porcupine. If not I suggest that you get both Melafix and Pimafix and treat them as directed for 7 days. You can use both at the same time, which is good. These will treat almost any illness that they might have. I lost 2 guppies the same way (lumps), and it turned into dropsy before I could treat them and they died. I still have 2 more and they were immediately treated with both and all my my other fish too and all are doing great.