Calories burned calculations— using avg heart rate

So I have a heart rate monitor I use to find my average heart rate during exercise. Then I use to equation found on livestrong's website—

Calories Burned = [(0.4472HR+ 0.1263W + 0.074A - 20.4022) / 4.184] * T

Which normally gives me a relatively reliable amount of calories burned. But today it told me I burned nearly 2000 calories playing tennis this morning for 2 hours, and it just doesn't seem right!

Does anyone know what equation is most reliable? I mean, MFP says I only burned 800ish calories. More than anything, I jus don't want to go into starvation mode by not eating enough.

And my HRM says I only burned 55 calories, so don't tell me to go by that haha.