Could a nerve problem be causing such symptoms? such as left shoulder pain when breathing deeply and lying down, left arm tingling at all times.

Yes Possibly: Left arm pain and tingling certainly sounds like nerve pain, but the deep breathing and lying down also raise the possibility of other sources. Although you are very young, left arm pain can also be radiating pain from the heart or chest area. Start with a visit to your regular physician to narrow down the possibilities.

Yes: As tingling in arm is constant, and worsens with change of position, my best guess would be nerve compression within the shoulder. Breathing deeply is like the positional change, it puts further pressure of the nerve as the lungs fill with air and compress anatomy upwards into the nerve compression site. See an orthopedist.