I had this too (with Concerta specifically). I hadn't been taking it for long at that point, a month or two I think? At the low dosage of 18mg x 2. I wasn't getting the therapeutic benefit despite having the exact same problems you described. I had 1mg of daily clonazepam, but it didn't help at all. Knees, feet, fingers and elbows were the worst for me and it's not just a simple ache or soreness, it was proper pain that felt pretty serious to me. Also had issues with vasoconstriction.
Dystonia from MPH is a new one for me, and according to brief research it seems to occur most often in young children and people taking other psych meds with MPH, though in rare cases just MPH alone has caused a dystonic reaction. Are you also taking other dopaminergic drugs, anti-psychotics specifically? SSRI's? Venlaflaxine/efexor? The exact causes of these reactions aren't really known (dystonia etc), with MPH there's usually a delay of few weeks before they begin to manifest and once they do, you start experiencing intense symptoms within hours of dosing IIRC. Best advice I can give is switch to a different stimulant medication if you need one, as you really shouldn't take MPH when the side-effects are this serious I'm afraid. I have had one episode of acute (cervical) dystonia in my life for other reasons and holy shit, never again!
Just my personal experience as a patient.