You have certainly had alot going on...I am sorry.
(1) you're not having an exacerbation, your body is simply saying enough and with some rest your symptoms will decrease.
(2) You are in fact having an exacerbation
The MRI is a great diagnositc tool, however, the MRI is not good at showing if you are having an exacerbation. You can be feeling fine, have a MRI and it shows active lesions. You could be having an exacerbation and the MRI shows no changes. MRIs do not tell the whole story of this disease nor does the MRI necessarily correlate with how you feel.
There is no way to avert or avoid having an exacerbation. You can take precautions and try to avoid the things that you know makes your MS worse. Unfortunately, we are not in control, MS is.
Although you are using Avonex you can still have exacerbations. The Disease Modifying Drugs (DMDs) are to HOPEFULLY decrease the amount and severity of exacerbations and to HOPEFULLY slow progression.
The question becomes - would you have gone 3 years without an exacerbation if you had not been using Avonex? Unfortunately, there is no answer. You might have had the same outcome even if you weren't using Avonex or one of the other DMDs. Or, you might have had more exacerbations had you not been on Avonex..
Managing exacerbations: rest, mild exercise as tolerated, medication that can help with the symptoms and/or steroids.