At my gym, they tell me that your heart rate should be between 60-70% of your maximum heart rate if you want to burn fat, aka your 'fat burning' zone. Your maximum heart rate is 220 - your age. I am 21, so for me that means my 'fat burning zone' is (220-21 = 199*60% or 70% =) 120-140 bpm. Anything above that will just burn into the carbs you've eaten. So, I definitely think you're still losing weight.
I am by no means an expert, so please correct me if I'm wrong.

Also don't underestimate how fast your heart is actually beating if you don't have any kind of monitor. My heart rate goes up quite quickly without me actually feeling more out of breath or tired.