How long have you been doing that particular workout in that particular way? If it's been 4-6 weeks, then yeah, you meay be reaching the point where your body has adapted and gotten fitter. If that's the case, it's time to switch it up - go faster, go harder, do a different type of exercise.
Sometimes you just have a really great workout where you feel like you're on top of the world and feel like a million bucks on top of it. But if you've been doing that workout for several weeks, then it's possible you may need to change things up a bit.