Weight lifting doesn't burn all that many calories directly - after all, even a 20-30 minute workout only involves a few dozen reps.
The calorie burning effect is indirect. One well known study (which I can't find the link to at the moment) put the effect of a 12 week program of lifting at about 150 calories per day (even on rest days), with about 50 calories coming from increased muscle mass, and 100 calories from making existing muscle mass more metabolically active.'
But you can't attribute this to a specific strength workout - it is really the effect of an ongoing program of regular training on your overall metabolism.
HRM's are calibrated for aerobic exercise, and will not produce accurate results for anerobic strength exercise (a different bio-chemical process).
But it is unlikely that 150 calories is producing an 'undereating' plateau on their own - this is more likely to occur through a combination of an extensive cardio schedule combined with minimal intake.
You may find the Plateau Busters series of articles of interest www.sparkpeople.com/resource/fitness_artic