It could be many reasons, but it is hard to say for sure that your lungs have adjusted/ or healed/ or cleared themselves of residual tar etc. It could be many reasons, but we need some sort of details. Are you building your own
coils, are you sure your
coils are clean, is your wick dirty or burning. Vapor does make most first time users cough as they usually take to large of lung hits. Are you mouthing your vape? MOst say its easier to take the vapor into your mouth and slowly inhale till one gets accustomed to it. Vapor is also much more humid and dense/ but also dries out your mucous membranes and lining of your sinuses and throat etc.
Make sure you drink lots of water, make sure your equipment such as wicks and fillers are clean and not burning. Since we have no idea if you are using clearos, cartos, RBA, RDA or what resistance, its hard to say.