I know this is an old post, but I thought I may be able to help someone out.
It may be an anxiety
attack. Although I quit a few times, I smoked almost daily for 5 years and never had a problem, when I was 19 I started having symptoms: racing heart, tight chest, light-headed, and a general "odd" feeling every time I smoked, so I quit. A while later, I had the symptoms again while I was straight/sober. I went to the hospital a few times because I thought I was having a heart attack
(doctors couldn't find anything wrong), but after some research I found out that they were Anxiety attacks.
I don't do it that often anymore, but I can also smoke now while I'm drunk/drinking. Alcohol shuts off the part of your brain that deals with fear (along with a lot of other parts), that is one reason people become alcoholics, it's called self medicating/worst reason ever to drink.
I learned to "see" the symptoms of anxiety building in me before I have an attack (twitching eyelids, hair on my body standing on end, tense muscles, etc...), and have been learning to keep myself calm and relaxed.
This is just my experience, and my problems are getting worse (pre-hypertension
with an extremely low pulse), plus I still get anxiety attacks. I'm sure that if someone would have figured it out back then, I would be better off now.
Don't mess around when it comes to your health bro, go see a good doctor (or 10).