Hello Cough,cold and rash followed by fleeting swelling of joints in this age can be due to infection(lymes disease,streptococcal infection),rheumatic fever(post streptococcal infection) or vascular disorder like Henoch- Schonlein purpura.There can be other uncommon causes for this condition. The exact diagnosis can be made by clinical examination and few investigations. The management depends on the diagnosis. I would suggest you to consult a pediatrician for proper clinical evaluation and management accordingly. Regards