Your feelings are pretty normal especially if you get put into cardioresp. straight out of school. That said you should have done some useful clinical placements in that area.

We have a couple of really useful texts on our shop

Physiotherapy for Respiratory and Cardiac Problems, 4th Edition - Adults and Paediatrics

Respiratory Physiotherapy 2nd Edition - An On Call Survival Guide

The Physiotherapist's Pocketbook 2nd Edition

Chest X-Ray Made Easy, 2nd Edition

Most importantly talk to your boss. Tell them how you're feeling as to off load it in a professional manner should ease your mind. Rest assured a couple of months in and you'll be feeling like you've been working there for years.