Only way to truly know what happened to Thumper would be a necropsy, without that, there just isn’t a way to know and if it was something contagious, likely your other bun would have it. All vets charge for a check up, no vet I know of does free medical care, they wouldn’t make any money that way – but check ups shouldn’t be too expensive, my vet only charges $30 for a check up. Check the area for anything that might cause an environmental issue – anything toxic, or that they might chew that could cause an impaction, etc. Wash everything down and disinfect (safely). Keep an eye on her for symptoms – not eating, change in droppings etc. But really there isn’t a way to know what steps to take without knowing what happened to your first.
As for her weight, most young buns are thin. What is her diet like at the moment? If it is a good diet, probably she is fine in that regard. A checkup would tell you for sure though.
Any pet is taking on financial responsibility of their vet care and a check up is the least of this cost. If money is tight, a good idea is to start putting a couple of dollars away when you get paid into a savings account for vet funds, that way you never have to get hit out of bill money etc.Or look into insurance for your bun.