Help figuring out a problem

Age 46, woman. Exercise 6 days a week for two months. Mostly gim fitness classes. Can't do more than 15 min low intensity trx (it is me and grannies). It is not difficult for me, but my heart wants to get out from my body. If I don't stop I feel like going to faint. And it take a lot of time to recover. Doctor did heart endurance test and said I'm totally fine. I can do reformer quite well. But even if I do just lunges I get the same reaction as someone will get after running like crazy. Any advice will be really appreciated. Thank you