I'm experiencing this pain on the left side of my chest right below the breast. It's a bit worrying because it gets slightly worse when I take a deep breath, and now I can't even sleep on my left side because it hurts too much. Could you let me know what's going on or what I should do about it?

It sounds like you may be experiencing musculoskeletal chest pain. You can try taking over-the-counter pain relievers such as ibuprofen (Advil) 400mg every 4-6 hours as needed for pain. Additionally, applying a warm compress to the area may help relax the muscles and reduce pain. Make sure to rest and avoid activities that worsen the pain. If the pain persists or worsens, please seek medical attention for further evaluation.