Hi and welcome,
The reality is that what you are experiencing is unlikely to be caused by a neurological condition like MS and more likely to to caused by something else...
You are acknowledging your medical condition (general anxiety) will at times make it difficult for you to know if something you've experienced is real or imagined, so there is the possibility your general anxiety has developed into being more specifically to do with your health.
I'm not saying it is or isn't, just that if this is already identified, the possibility is higher that your anxiety could be making you hyper focus on to your health and you are inadvertently feeding your health related anxiety further with your unchallenged obsessive behaviours ie googling symptom, reading about MS, self checking etc. I would highly recommend you consider contacting your psychiatrist or psychologist and proactively seeking additional mental health directed help and support in this moment of your life.
Hope that helps......JJ
btw Pain caused by neurological damage from a condition like MS wouldn't actually feel better with exercise and worse without, shin pain more specifically is usually the result of chronic overuse, physical activity overloading the structures such as your bone, muscle and tendons so it's possible this started from something your doing whilst exercising, it's an idea to consider at least.