Pain in left shoulder/upper back. Sometimes with numb/tingly feeling in arm. Very frequently carrying toddler on left side. How can I treat this?

Carry on other side. Rest your neck and left shoulder as much as possible. Would consider getting a cervical mri.
Chiropractor. I had a lot of shoulder and neck and back pain after having a baby. I found chiropractor and massage therapist were super helpful, in addition they give you exercises to help stretch out those muscles that are impinging on your nerves. Worked wonders for me. In meantime, warm heat, like an electric heat pad, can help, as well as warm bath and ibuprofen.
Chiropractor. Sounds musculoskeletal. Would probably benifit from chiropractic evaluation.