However, some medications used for symptom relief can cause weight
gain, specifically Neurontin (Gabapentin) and Lyrica (Pregabalin). Steroids
can have this effect as well.
Many MSers put on weight because fatigue makes them lethargic, so they easily can become couch potatoes. That doesn't sound like you, though, as you get an admirable amount of exercise. I suggest you check with your primary doctor about potential causes of your weight issues.