This is a very common phenomenon in vaping. The first cause is long-term irritation from smoking, which makes your throat too sensitive to the nicotine in the vapor from your e-cigarette. After smoking for several years, tobacco causes a lot of damage to the larynx. Unlike tobacco cigarettes, e-liquids contain no additives to hide the irritating effect of nicotine. When you first start using your vape, you need to break down your vapor intake by directing the vapor into your mouth and only then slowly into your lungs. Just as you would with a conventional cigarette. If the phenomenon persists, you can lower the nicotine strength or use nicotine salts, which are virtually non-irritating and allow you to vape at high nicotine strengths. Your equipment may be unsuitable, causing severe irritation. When you start out, make sure you use resistance of over 1 ohm and avoid all clearomizers with direct inhalation. The vapor is often too strong and above 6mg of nicotine the vapor can become very unpleasant and cause coughing.