A few thoughts
If you have been running harder/faster then usual, you might have heaviness in your limbs if you haven't recovered from your previous run. I also recall the run (not a race) a couple of years ago when my legs felt really heavy on the second tenth of a mile. That remains my fastest ever run at that distance.
I find that my legs feel really heavy, and my times are really slow, if I run after being on my feet for several hours (I usually run in the morning, but occasionally my only opportunity is after work).
You don't give your age or health status, but it could be a medical issue -- anemia, or circulation problems.
You could be *mentally* over-trained, and need a short break.
Good luck on figuring it out.
Robin