Did I read anywhere on here about swelling legs as a symptom?
I'm not aware of this being a symptom of DM.
I guess I would head to urgent care/ER as soon as possible to rule out a blood clot, and then contact your doctor to let them know about what you have been experiencing.
My experience with IBM is that the muscle disorder which is more advanced in my left leg "coincidentally" has a tendency to swell during the day (especially if sitting at desk for long periods) rather than my right leg.
I mentioned this to my doctor and his view was that the muscles assist the flow of blood back to the heart and as they have weakened to a greater extent in my left leg this could be part of the problem. I did undergo an ultrasound and physical checks and no clot was suspected or found.
Not really conclusive but the whole subject of IBM seems to be shrouded in mystery. Just another little bit of experience.