hello psi! think you've gone to bed now,
I get this some times, put it down to being right handed and that my right leg works harder, what do you think?
I also have problems in the winter sometimes from (i think) running on sloping pavements, but that's more in the sides of my legs,
There is a theory that it's minute tears that are stiffness, not lactate, I'm not qualified to comment, but this would fit with one-sided thingy, sure you'll get more informed advice tomorrow.