I had the same problem a while back, dunno why either. I just kept going all the same and eventually i started to lose weight. If your trying to lose weight more than gain in muscle size then you should change your routine a bit. Try more cardio work and change your reps. Your programme of 3 sets of 8 is manly for size, maybe try lifting a little less weight and go for sets of 12 plus. Just an idea...