Twinkle, well done.
Shelly-lynn Florence Glover, who is an eminent Americal running coach, suggests that of every five runs, one will be a real stormer, three will be OK, and one will be a bummer. You had a bummer. It happens. Look upon it as having got a bad run out of your system. I think bummers are quite a random phenomenon, but there are plenty of theories about the cause (fatigue, nadequate fuelling and suchlike).
I've had two bummers this week (I'm one of those post-FLM leaden-leggers that Marzipan Treefrog mentions), but a rather good run today.
Keep up the good work. Happy running!
Cheers, V-rap.