Yeah, unfortunately many people don't learn about stuff like that until after it's too late. Places like petco and petsmart dont tell you about that stuff when you buy an animal.
It sounds to me like it may have been some sort of sickness passed from the new frog to the others. I suppose coincidence is possible, but the timing makes me suspicious. You said the legs were red on the female, if they were very red it could have been Red Leg Disease.