What you're experiencing sound like transitional PVC's. These are premature contractions of the ventricles triggered by a response to the body's need to increase blood pressure. These are common for many of us, I get them going upstairs or when I'm warming up to exercise or even when I stand up from sitting.
In the presence of a normal heart structure they are almost always nothing to worry about, but if they are new you should mention it to your doctor.
Hope this helps,
Jon
It is good you were checked by a cardiologist, but if you have new symptoms it may be good to see that doctor again.
Your anxiety noted, that could be a (or the) driver. I understand that given anxiety just thinking about your heart beat could trigger rhythm problems.
I hope your therapy for anxiety helps, your trying to not think about what your heart is doing and being optimistic (i.e., you doctor told you nothing is wrong) may work well with your therapy.