hm... with 12 hours time in between that's a tough one.

i would think you taxed BOTH your muscles and your cardio system and you just weren't recovered fully in 12 hours (up to 2 days for full recovery)...

although normally i would expect more muscle fatigue and reduced performance and an ELEVATED HR, so not exactly sure -- normally the inability to get your HR up is from lack of water/nutrients/energy... i assume you ate and drank a fair amount while at work, like at lunch for example?

but i think it sounds normal enough if you really pushed yourself that hard and harder than normal.