I Am A White Female 68 Years Old And Go

I am a white female 68 years old and go to the gym periodically and do the treadmill or exercyle. I do each at a fairly low level but the machines shut down about 1/2 to 3/4 of the time saying I have exceeded the maximum heart rate. I mentioned this to my dr. at my last physical and she didn't seem concerned but it continues to happen and never has happened to my husband. The gym says no one has reported having this problem. How concerned should I be about this?