I've Had Irregular Heart Rhythms In Recent Weeks - At

I've had irregular heart rhythms in recent weeks - at times it feels like my heart is pounding out of my chest even when I'm resting. I was just driving home thirty minutes ago and felt a tightness in my chest and a rush - I used a heart rate monitor app on my phone and measured 154 on two separate occasions two minutes apart. Ten minutes later, it was at 96 (again twice and two minutes apart), and is now bouncing back and forth between low nineties to high nineties.

I'm a 24 year old male who only occasionally smokes and does drink rather too often. I don't exercise regularly, though I do run a few miles every so often.

I'm still feeling tightness and discomfort in chest. What should I do/think?