We cannot tell you what is wrong with you nor tell you how serious without examining you? Much less, if we are not doctors.
It's best that you consult a cardiologist
who might recommend you to undergo some tests. Ultimately, you may need to do an angiogram (insert a minitube into your vein and inject a dye and scanned by some x-ray machine) to see if there are any blockages in your cardio-vascular system. That's what happened to me after I underwent a stress test
i.e. run on the treadmill and have the probes check my heartbeat pattern. The stress test showed some irregularity in pattern which led to my cardiologist recommend angiogram. I was given a clean bill of health i.e. no blockages! (yay!). Quite a few doctors I asked says that the stress test is not 100% accurate (even going as low as 50%!!) but at least it led to me doing a more accurate one. Now, I feel more at ease.
So, my advice - Go see a cardiologist...NOW! Before it is too late.
Send me an email to let me know how it went. All the best.