I Am Having Terrible Heartburn Which Is Highly Unusual For

I am having terrible heartburn which is highly unusual for me. I never get heartburn and I am wondering if I am having a heart attack or if it is a side affect from the doxycycline that I am taking for Lyme's disease. I am a healthy fit 54 yr old woman. The only other time I had heartburn was when I was pregnant 20 yrs ago.