It could be the PG, VG or the flavoring. Asthma inhalers have PG in them- used as a carrier. Some have reported a "heaviness" in their chest from VG.
Normally people that smoked 1-2 PAD begin @ 18 or 24 mg. For yourself 18 mg is far too high IMO. I would think either 12 or 8 mg. Are you getting any headaches, heart palpitations or dizziness?