Sometimes I think it is easy to talk yourself into thinking their is something wrong because an animal isn't acting as you expect but actually it's just way it is.
My first lambs were triplets, three tups, suffolks born in December and lambed inside and access to outside. You would think they would have been running about once they got but no. They hardly ever ran or skipped like lambs. Just ate, slept and chilled. My next, a single gimmer was a live wire from day one.
You have given the lamb a thorough examination and found nothing so I wouldn't worry, just keep an eye.