Hello and welcome to BYC! My very first chickens were ISA Browns, and I've had over 100 since then. I absolutely adore their personalities; they are so friendly, smart, curious and funny. It is normal for ISA Brown pullets/hens to lay big, beautiful, golden-brown eggs every single day. Which means that the fact one of your pullets suddenly stopped laying is not normal. Unfortunately their little bodies pay a very high price for such a high lay rate. It would be unusual for one of your pullets to already be having reproductive problems, but it is a possibility. Continue to observe her behaviors, and you can also do bi-weekly physical exams. As much as my ISAs are constantly underfoot, they don't like to be picked up, but you can do a wellness check when they are on the roost at night. If your non-laying pullet shows any signs of illness, don't hesitate to create a thread in the Emergencies forum and ask for advice. The medical experts on that forum are extremely knowledgeable and always happy to help.