I Have Had A Dull Ache At The Base Of

I have had a dull ache at the base of my sternum for 5 days now. Sometimes my mid back aches also with this pain. I have visited my GP and have a blood test tomorrow. I have had an ECG at my local hospital and it is normal. Blood pressure and temp also normal. I have taken pain killers but they work only briefly. I have asked 3 medical professionals and no one seems worried about this but it is worrying me. The walk in centre just told me to go home. What could this be? What is the best way to ease this pain?