You're overreaching a bit. When you go hard and get your HR up, that happens because your glands release hormones that make that all happen on your command. (sheesh, what crappy construction, oh well) But you get that, right? So your glands get a little tired. That can happen from a series of high HR rides, or it can happen on a long, strenuous ride, or it can happen just from continuous training stress. But you're just overreaching a little, which means you aren't dead yet.

So when you reached that hill, you got some mental burst, got the glandular release to which so many of us are addicted, and away you went. However, that's a bad sign, that you had to dig for it. Pretty soon, when you dig for it, it won't be there and then you'll have some serious resting to do. I've seen this scenario a many times in myself, where I'll be just beat during a ride, but something will come up and I'll attack and my HR will come up and away I'll go.

As other commenters have suggested, the scenario I outlined may not be true in your case. It could easily be something else. But watch out for it. The way most of us avoid it is to have 1 week in 4 that's an easy week, or for the ancient among us, even 1 week in 3. If you're tired, that good feeling won't come back with just one day off.

To combat this, because I'm terrible about going hard too much and too often, I got a recording HRM years ago, download to my computer, and keep track of my time in zones every week.