Tragus swollen what's the cause?

Otolaryngologist. This is dependant on several factors. Does it come and go? Is it painful? Is the skin above tragus red? I believe you need to be examined by an otolaryngologist (ear, nose and throat doctor) to give you a real answer for this question.
Swimmer's Ear. If it is also tender it usually means that there is some irritation or infection in the ear canal. Keep water out of your ears and if it doesn't get better, see your doctor for some eardrops.