SunnyGir says:

I was diagnosed with costochondritis at first, but then they figured out that it is chronic myofascial pain (which is very similar to fibromyalgia). You probably have a trigger point (a knot in the muscle) – most likely several if you have had this for some time. The trigger point is most likely in your pectoralis and intercostal muscles and the only way to relieve a trigger point is to put pressure on it (Use a broad tip marker on the pointy end – I like those fat sharpie’s and consider buying a THERACANE – it is a great tool for the back!). Your external intercostal muscles are responsible for expanding your ribcage when you take a deep breath. It will hurt when you press on the trigger point at first and lots of times they refer pain (you will press on the trigger point and feel the pain else where). The pain will be felt at first but then it should gradually get less and less. You have to work out the trigger points (consider seeing a massage therapist who does myofascial release – I tried electro-acupuncture but it was NO-WHERE NEAR AS EFFECTIVE as massage therapy) and you also have to stretch your back, neck and ribcage (do a pec stretch). Remember that anything affected in the back will also be affected in the front and vice-versa. To do a pec stretch (which stretches your pectoral muscles, sternalis, and intercostals) stand in a doorway with your arms against the door frame in an “L” position (and keep your bicep at shoulder height and lean forward. You should feel a stretch in your upper chest. To stretch the neck (re: sternocledomastoid and scalenes) sit in a chair with the feet flat on the floor and arms by your side relaxed. Tilt your head to one side and hold when you feel a stretch. then do the other way. Also for the scalene muscles in the neck tilt the head to one side while seated in the chair and then at the same time turn your head upwards to the ceiling and hold. Also you should try to avoid chest breathing as this will weaken your Sternocleidomastoid Muscle in your neck (it assists you when you need to get extra air when taking a deep breath) – Make sure you try to abdominally breath – even if it hurts! Also make sure you sit with feet flat on the ground and your back straight and not slouched as this can lead to developing trigger points in your back & neck in the scalenes (re: neck) and in the rhomboids and trapezius in back & other places. You may also have trigger points in the sternalis muscle (the very middle of your chest). You should consider buying the the book by Claire Davies called “The Trigger Point Therapy Workbook”. Trigger points can also create sattelite trigger points.
Good luck to you. I got my diagnosis from a Thoracic Surgeon, but you can also see a Rheumatologist.