It could be an allergy to PG or VG, although PG allergies are more common. PG also gives a stronger throat hit. You could try using more VG and less PG to see if that helps and if it does, you may want to go to all VG. I thought Magic Mouthwash was given to people with Thrush, which is an over growth of yeast in the throat and mouth usually brought on by use of antibiotics. It can also be caused by too much sugar. So it could also be from VG which is sweet and more coating as it is thicker, thus creating thicker vapor. It could also be caused by sweet flavorings. I'd start with comparing PG and VG and flavorings as well.
Finally as was mentioned earlier, it could be due to excessive heat. If that is the case, you can use longer attys, cartos, and drip tips. Attys can also vary in heat production as well. LR attys burn hotter to simulate 5v vaping on 3.7v pvs. Atty heats also vary by brands as well. If you find your problem to be related to attys, you should read up on ohms and then get an ohm reader to test your attys. On your 510 Ego, make sure you are not using LR attys if you think it could be heat related and especially since you just started using one. If you are not sure if they are LR or Low Resistance attys, just look on the atty for an LR either right on the atty or on the bottom of it. Joye makes 510 XL (extra long) attys that work on Egos and that would definitely help if you find that heat is the problem and add an extra long drip tip. Do not get metal ones as they heat up, anodized aluminum ones are better but the delrins are the most heat resistant. I hope this helps and the best of luck to you!