So how many rides do you have behind you? For what total distance?
As you do more you get better. It's way too easy to overexert yourself when you are just starting out as not all of your body is in the right shape to handle what the rest of your body is throwing at it. So take it easier and don't quit.
Also, ask a doctor about anything that concerns you to make sure it's not some red flag for serious health related issue. But as
Calsun said, doctors don't always know the particulars of how to discuss sports related issues with their patients. I never got a realistic answer about exercising at very high heart rates until I got a heart doctor. All the others just gave oblique answers.