Hi,
The erythema migrans rash is specific to Lyme--You can be diagnosed on that alone without further tests.
Lyme disease doesn't always cause the rash, but if you have the rash, then you have Lyme. Take a picture of the rash in case it goes away.
The rash can occur within days of a tick bite near the bite, or the rash can be secondary and show up much later (years even) and be nowhere near the bite.
The blood tests for Lyme are simply not reliable. If you just got infected, it can take several weeks before your body will make the antibodies that show up in the tests. If you've had Lyme for a long time, then your immune system may no longer be making the antibodies in amounts great enough to show up on the test. So the tests can fail for both early and late cases.
My advice is to take a picture of the rash and show it to a doctor. The treatment for Lyme is antibiotics, which can vary a lot in price but some are inexpensive.
I do not have MS, I only have Lyme. You're welcome to come and post on the Lyme forum here on MedHelp:
http://www.medhelp.org/forums/Lyme-Disease/show/148
Take care.