Oops - I'll try again, apologies if it now posts more than once - no idea what I'm doing wrong.
As BB and Sea Angler have said, it seems that it's not an uncommon problem for people with Parkinson's but I'm no neurologist and there are probably other causes. That said, PD UK does have an advice sheet on cramps and dystonia if you haven't already seen it and wanted to check it out.
For me, it was one of my first symptoms and predates medication so isn't linked to 'off' periods - and it doesn't always respond to medication and it's a complete pain, literally and figuratively, however it's been easier to cope with since I've known what it is. I hope you get some answers soon, I was given a diagnosis after a very short period of time following my first appointment but I do remember that awful feeling of being in limbo. Take care, J