odd HR spike

I went out for my LSD run this morning. I setup a route for just over 9 miles, which is the longest run I've ever done(I've hit 8 & 9 a couple times, so it's not completely unprecedented). The run was going decently well - at least for a run where most of the first 5+ miles I was going into a sustained 10-15mph headwind - and I was keeping my HR somewhere in zone 3 at a 10:30-11:00 pace. For some reason when I hit about 5.5-6 miles in my HR jumped 5-8bpm and wouldn't drop back down. I can't quite figure out why; I wasn't picking up the pace and the terrain was relatively flat. Even the minor hills were with the wind at my back. But I couldn't get it to drop down even when I slowed down. Now I've done longer distances than the 5.5-6 miles when the HR jumped up, and it wasn't all that hot outside. It also bears mentioning that I didn't feel suddenly more tired at the time either. I also had water with me, so I don't think it was a dehydration issue. Any thoughts on why this would happen? I can understand the HR creeping up over time and that wouldn't really concern me, it's the fact that it was sudden(like within the space of a minute or less) that makes me wonder what was going on.