It can take a while to show up. Ich goes through a couple of different life stages before it becomes infectious. It really is a fascinating critter from a biological standpoint. Not so fascinating when its on your fish. That's why some shops quarantine fish. They wait for the fish to develop ich and then the copper in the quarantine tank zaps it. Sorta. As I recall, on the free swimming stage is actually killed by copper. Neverthe less. My point is that like any disease, there is an incubation period. Infectious lesions don't appear over night. The stress that started the infection could have occured weeks ago.