HRM won't record any calories burned other than walking

Help me! OMG, okay, so I've had my HRM since christmas, and I had no problems with it, as I was only using it for my walks/jogs. I loved it, thought it was great until I started doing yoga. I'm doing power yoga, as in Jillian Michaels Yoga Meltdown, and I am sweating buckets with this dvd. Heart is pounding, I can feel the pounds shedding, you know? But my HRM will say I only burned 7-10 calories. I know it doesn't work with strength training, and you need to get your heart rate up in order for it to count, and my heart is definitely beating up there! So I don't get what's going on?
Anyone else have this problem?