Post infection you can have phlegm accumulation inside your chest at times. Which is gradually get better on its own. Now since you are already feeling better, nothing else to be done. Steam inhalation twice a day for a couple of days will help get much better. Pain maybe due to post infective costochondritis
Next Steps
chest xray
Health Tips
if pain not subsiding can take
C Rejoint twice daily for 5 days
T. raciper 40mg once daily for 5 days