This is just a guess but has Duncan been checked for Lyme disease? Bella developed Lyme disease a couple of years ago and her first symptom was a limp. I thought she had pulled a muscle running in the yard. It was right before she started on Frontline or received the vaccine. We caught it in time and she was put on a vigorous dose of antibiotics and is fine now. Ticks can be bad in our part of the country (I am from NJ too) so that might be a place to start. I hope whatever it is, is fixable and nothing serious. (((hugs)))