I Had A Heart Attack 3 Months Ago And Had

I had A heart attack 3 months ago and had one stent on the OM. I am taking cardiac rehab (treadmill, bicycle, and eliptical all while hooked up to wireless heart monitor) which is not causing Me any problems. I was released to go back to work with no restrictions. Now when lifting or carrying anything more than 15-20 pounds for A very short time I can not seem to breath and have to stop and rest for A few minutes. Is this normal or do I have more problems that need addressed or is it just too soon after the heart attack to be able to do much. I am not over weight, I no longer smoke, and I am 56 years old.