I am a 47 year old female, and have been exercising moderately for most of my life. The past couple of months, I get winded very easily. It is not tough to breathe, like there is something sitting on my chest. Rather, after running for only 3 or 4 minutes I have to stop and walk because I am panting so hard and my heart is beating very fast. I recover pretty quickly and start running again after walking for a minute or so, but the cycle repeats. Any idea what might be wrong?