Some flavors vape a bit harsher, higher nicotine vapes a bit harsher, lower resistance vapes a bit harsher (lower resistance for a given voltage PV = more flavor/vapor, warmth), some people have sensitivity to PG, VG, or flavors
As a brand new vaper, you're likely just getting the typical new person harshness -- you need to drink lots of water, and the inhale for a PV, for many people, is not the same as for a cigarette. Generally, try to draw the vapor into your mouth, then remove the PV from it and finish the inhale into your lungs. Many people starting out will try to directly inhale, short, hard, puffs like a cigarette, and get harshness and/or coughing.
Second option, if this continues (doesn't lessen noticeably) for more than a couple days, after drinking lots of water and working a bit on your inhalation, is a higher resistance cartomizer. If you have a 2.5 ohm cartomizer, try a 3 ohm.
Good luck!