Pertussis is a severe and prolonged cough, and I've seen patients who have actually broken ribs due to the coughing. Obviously, I know little of your health history, but you're more likely to break a rib under such circumstances if you're an older female with possible osteoporosis. Sometimes, people have inter-rib muscle spasms that last for months after pertussis. I can't say if this is the answer to the problem, but it might be worth discussing with your doctor if a chest x-ray is appropriate. If it's gradually improving, I'd just do some nightly deep breathing exercises to expand the chest wall, and monitor. As long as it gets better, there's unlikely to be anything serious. Unlikely a hernia (not that high up and they don't pop).
See a chiropractor and get an x-ray, your heavy coughing could have dislocated some vertebrae in that area, if so, nothing will take the pain away except a chiropractic treatment.
Actually, I highly advise avoiding chiropractors as they have a tendency to break ribs (seen and treated many caused by chiropractic manipulation). If you want to try manipulation, I would seek out an Osteopathic Doctor (they have a D.O. after their name instead of an M.D.) as their approach is much more gentle. They all require a chest x-ray regardless of who you go to. Also, you can't dislocate vertebrae. :) You can, however, dislocate where the rib attaches, but that generally causes crippling constant pain.