Why am I so winded?

So, i'm doing my posture exercises, the same ones I've been doing for about a month. Working on the first exercise--spinal mobilization--all it is is standing with your hands behind your head and bending from side to side. But I'm too tired to stand, so I'm kneeling...and my heart is pounding so hard that I have to rest after each 30 second rep. I'm also sweating and breathing hard--like I just ran up a flight of stairs. This is odd, but not as infrequent as I'd like it to be. I've been in good shape my whole life, so it's disturbing when this happens to me now. I don't know what the cause would be only that this is a common symptom of Fibromyalgia.