This does not sound trivial. take her to the vet now and at the END of the visit explain your financial situation and give them a $20 deposit and tell them you will send them $10 a week until it is paid off. If they don't like it tough. (and go to a different vet next time)!
did your vet do the full comprehensive stool panel? the one that costs $250?
also, new foods take up to a month to settle in. I/D works well, as does lamb/rice mix
good luck