Inflamed sternomanubrial joint

Have chest pain since January and still no relief. After ruling out the heart (stress test), esophagus(upper gi), lungs(X-ray) and other organs (ct scan with dye)I had a bone scan that showed increased activity in the sternomundabrial joint. I had to follow up with a mri that revealed mild marrow endema. My rheumatologist has me one 15 mg of prednisone. I still feel discomfort in my sternum and left chest when I turn the wrong way or sometimes cough. The pain or nerve irritation in the chest peaks at work between 9-3 pm then subsides a bit. Doctor feels I have tietze syndrome but not 100%. He states I may have psoriatic arthritis but again not sure. Because the prednisone is not taking all the pain away he want me to try methotrexate after I wheen off the prednisone. This ailment has really got me worried. Your thoughts?