When i laugh hard, i develop phlegm in my throat. i had a sinus infection and been using nasal spray till now. why does phlegm occur after laughed?

It moves around/out!: I'm speaking from personal experience only. Any big movement in the upper respiratory tract - laugh, cough, nose blow, big gasp, etc. moves the existing mucus and can break off sections of it allowing it to be expectorated (good!). It doesn't develop at that time - it becomes free to get out.

Several: Post-nasal drips from allergic rhinitis and sinusitis can cause the cough but one needs to rule out asthma which often manifests just as a cough without wheezing or noticeably breathing problem (except with exertion).