I believe a stress test on a TM lasts about 15 minutes and is done in 3 stages. It's been a long time since hubby had one as now he gets the Nuclear one as he can't do the TM anymore.
The treadmill is started at a relatively slow "warm-up" speed. The treadmill speed and inclination are increased about every 3-4 minutes. Of course your BP is recorded during this every so many minutes. Could be your BP or HR went too high or as Vyking said, they saw something of concern especially seeing you said you were so out of breath.
Seeing you had the Tilt test and the Echo plus the stress test I guess they covered all the bases. Hopefully your results are ok.
Please keep us posted about your results and best of wishes..JJ..