As mentioned before, I'm trying to run fairly regularly at the moment (got a few Tough Mudders planned as well as my sort-of-yearly Half Marathon). I went for one run while at my parents', nice and easy, had a few quiet days in between (driving home and busy at work) but ever since I started back again every run was so much harder than before. I was out of breath and even had to slow to a walk in places I've been running past for the last few months.
I could not figure out why that was. I thought I'd maybe injured myself, blamed the busy nights of work with little to no sleep, thought about gravity fluctuations, hormones, the weather, shoes that didn't work properly on English soil, anything, really (mostly while running, it's very good for random and weird thoughts).
Until I remembered that after the run at my parents' I went and gave blood. Yes, maybe half a litre of blood less might influence speed and endurance just a tiny little bit...
In my defense, my regularly giving blood and the times when I was doing regular cardio workouts have never overlapped before.