I know it's scary when you see blood the first time. Now, take a deep breath - that's better.
Yes, worms could be the reason and there there are other reason. Just keep an eye on them, whoever it is he/she is not in pain. You are definately doing the right thing take her to the VET.
The other way to weed out the cat with the problems is to isolate one cat at a time in a room with it's own litterbox, food & water and watch for stool. That way you can easily determin who has the problem.
Good luck and heads up, your kitty will be just fine.
Oh, and if your VET continues not to be concerned about things like this, in my humble opinion, it's time to find one that is.