sudden rise in heart rate

HI all. I experienced something new for me this weekend and wanted to see if others have experienced the same and if my self diagnosis is right. On Saturday I did a long brick (for me) 50 mile bike followed by 3 mile run. I felt really good on Saturday and had a really strong pace on the brick run. Then on Sunday I headed out for my long run of 10 miles. I was cruising (or trudging) along in zone 2. Things felt sort of tough from the get go but kept a decent pace and my HR in check until mile 8 when HR started rising. nothing really changed that much in terms of actual or perceived effort by HR climbed @ 30 bpm (20%). I'm guessing I wasn't recovered enough from Saturday's effort and my body was calling a time out? I've been notoriously poor at re-hydrating / recovery fueling and something I really need to improve on. Note on my long run I brought along a fuel belt with some Gatorade endurance and water. it was first time using Gatorade endurance but wouldn't think that would be the culprit. thanks in advance for any intel.
-Patrick