I would think the doctors would get you in for a echocardiogram very soon. I had four of them in about a 45 day period a couple of months ago. I first noticed as you describe. I was walking fine but running up the stairs or climbing up and down ladders was when I first noticed a problem.
age 70, dx RAEB-2 on 11-26-2013 w/11% blasts. 8 cycles Vidaza 3w/Revlimid. SCT 8/15/2014, relapsed@Day+210 (AML). Now(SCT-Day+1005). Prepping w/ 10 days Dacogen for DLI on 6/9/2017.