First off, make an appointment with your regular doc and have a full workup. ERs mainly fight ongoing emergencies, they don't look at the possibility of future problems.
Second, watts? Ohms? Has anything changed recently? (settings, juice flavor or brand, etc.) How long have you been at 3 mgs?
vaping anything besides nicotine? Are you currently stressed? Maybe vaping like a freight train on occasion? Any changes in other parts of your life that you can pinpoint?
When I get palps I'll take a look at what I've been been doing for the last day. Usually it was vaped way too much, but sometimes it was crappy sleep the night before. Anything over 60% VG also makes my chest heavy and I recently ran into a particular juice flavor that my lungs just don't appreciate. Once I switched flavors I felt better. The same with putting down the mod. Additionally, with a change of equipment I sometimes have to alter my nic dose. As an example, I bought a new RTA not long ago and even though I'm still at the same wattage and ohms as the prebuilt coil system I was using previously, I still had to drop my nic from 3 mg to 2.
While you're waiting on that doctor's appointment try making some changes, one at a time: if you're doing too much hand to mouth, cut back on the number of puffs you take. If that doesn't work, try going with 0 nic for a few days. Look around and see if you can find a juice brand that has a 50/50 mix instead of the 70/30 you've been using. Try changing flavor profiles. You might even switch to flavorless to see if that could be the issue.
Good luck figuring this out. If all else fails, there's always other things to switch to other than smoking if you don't think you can completely quit the nicotine.