Hi can anyone help.. I am a 27 year old female, I exercise regularly and have a healthy diet.

I find it very hard to take a deep breath, when I manage to breath deeply I get a stabbing pain in my heart. I also feel my heart beat is irregular for the past 2 days. I wake regularly to catch my breath and my heart is racing.

I don't drink and gave up cigarettes over 3 weeks ago (smoked less than 5 a day)

Your lungs are still healing from the smoking. Another thing is if your heartbeat is irregular then you need to see your doctor to get checked out. Are you taking any medications?
Another thing is there is the possibility of MVP. And now that you have quit smoking its showing itself.

Ref:

Mitral valve prolapse - MayoClinic.com (http://www.mayoclinic.com/health/mitral-valve-prolapse/DS00504)

SOB (shortness of breath) is a reason to go to the emergency room. Please get checked out ASAP.

J_9 is right. If you have trouble breathing you need to go to the ER or Urgent Care, so they can do some testing and find out what's going on.