It is probably a herx. Lyme Disease is known to make the knee swell. When one herxes, new symptoms, old symptoms and your regular symptoms can come out. I had a few new ones that were pretty scary.
2003: symptoms started;panic attacks,anxiety,heightened senses.
2007: Diagnosed with Lyme. Didn't start treatment.
2008: symptoms came back with a vengence.
2009: Diagnosed with Lyme plus co-infections....Babesia and Bartonella.
Illnesses:-Chronic Lyme Disease,Hashimotos Thyroiditis, Ulcers, Hypoglycemia
Medications:-Currently on 1000mg Amoxicillian,Ultram,Thyro Complex 60mg,Probiotics and supplements