sorry you find yourself here, asking these questions. I hope this is only a temporary set back. I can't imagine that changing your infusion schedule one week would have caused your situation, but I honestly don't know.
This board seems to be more active, with more Tysabri users. Maybe they can help you.http://www.msworld.org/forum/forumdisplay.php?f=80
Your MRI will provide more answers.
If your MRI shows no new lesions, that will answer your question on whether or not tysabri is stopping the immune/inflammatory reaction.
But also ask how your gray matter looks on MRI. Is there any atrophy? How is your third ventricle, your thalamus and your brain stem? Have they changed since your last MRI? This is another area of investigation, and you don't need a special MRI--just ask the techs or your neuro to compare to your last MRI.
Hang in there. Do all you can to support your health with good sleep and nutrition. Keep moving, any way you can.
Let us know how it goes--