Hi Jenny,
It sounds like this is not something to worry about at this stage, but worth keeping close attention to make sure it doesn’t get worse. I can’t really tell what it is, but It could be some light edema, which is by no means dangerous. If that is the case then having your legs slightly raised with a pillow while you sleep will likely help, as well as raising your leg for a few minutes after a strenuous walk.
Shlomi