I have these two cats that use to be very sweet to eachother but i after i gave one a bath the other cat has been hissing and growling at her and i don't know what to do.
It should sort itself out as the cat starts to smell normal again. In bathing him or her you have taken away his or her normal scent. If you used scented products it might be worth a second bath to remove as much as possible of them. Going to the vet can produce similar problems for the same reason - the cat smells different.
Am curious about why you bathed the cat, and what sort of coat it has.
My two have issues after going to the vet. Even when they go together! Like OrientalSlave said; it's the changed scent. I usually put a dab of vanilla extract on the back of each of their necks to help them start smelling alike again. It can take a couple days sometimes for everything to be completely back to normal.
It's definitely because of smell - cats are very tuned in to smell and anything new or unfamiliar is like pulling the rug out from under them - very destabliizing. Like Oriental Slave, I'm also curious - why did you bathe your cat?
Either way, keep a close eye on their behavior and interactions. Make sure the more aggressive cat isn't intimidating the other cat to the point where the clean cat stops eating because of the bullying. Happened to my kitties and I didn't notice it immediately that it went beyond the normal hissing. I usually feed kitties right before work, and I came back to empty dishes so I figured they both were eating. I noticed on a weekend that one of the kitties was 'rough housing' and it was putting off the other cat from eating!!!!