A mild nagging tenderness / fullness (and ultrasound being normal) is likely to be myofascial pain syndrome. These are hyperirritable tender spots in palpable tense bands of skeletal muscle. Trigger points may develop after an initial injury to muscle fibers. This injury may be a noticeable traumatic event or repetitive microtrauma to the muscles (like cough for some duration). The trigger point causes pain and stress in the muscle or muscle fiber. Unless this is treated actively, this may persist for long time. The treatment is following;
1) passive stretching of the affected muscle after application of sprayed vapocoolant.
2) physical therapy; simple muscle stretch, augmented muscle stretch, post-isometric relaxation.
3) deep electrotherapy; iontophoresis, phonophoresis, short wave diathermy, electrical stimulation, high voltage galvanic stimulation, biofeedback.
4) local analgesic patch / ointment / spray
5) anti-inflammatory analgesics; Ibuprofen (Motrin / Advil)
6) ischemic compression therapy; pressure on the points
8) steroid shots
The other possibility is Costochondritis; it is inflammation at the junctions of rib and cartilage. Local analgesic ointment, or spray, chest brace and warm compresses with Ibuprofen is helpful. Another possibility is muscle sprain. Please read about the costochondritis;
Please feel free for your follow up questions.
I would be happy to assist you further, if you need any more information.