If this was the first time you’ve felt the dead legs than it is unlikely you are overtrained. However it is wise to be cautious and you did the right thing by dropping Sunday’s run from the schedule. Your legs are speaking the truth and regardless of your anxiety about “falling behind” they can only handle what they can handle and no amount of forcing will make them respond any better. When you’ve got those dead legs you must pull the plug on training and try to get them back to normal ASAP. Double down on recovery tools like rolling your quads. Do some easy swims if at all possible.
I can’t tell you exactly why your legs felt like this. Normally this would come from ramping up training too soon, too fast. I might also have to do with life stressors. But whatever it is needs to be dealt with aggressively.
If your legs bounce back in 2-3 days then it was not over training but more likely over reaching and you dodged the bullet and can learn a lesson. If they are still wasted feeling week later then you do have a more serious problem. But the treatment is the same. Rest till they feel better using every recovery trick you can throw at them.