I'm gonna take a stab in the dark and say that it's a copy.

It will probably have a sticker inside visible through the right hand F-hole that says something like "Antonious Stradivarious Cremonensis Faciebat Anno xxxx", but that just indicates which year the instrument copied is from.

Stradivarius' instruments are basically all accounted for, and no more 'lost' instruments have been found for generations.

From memory these sorts of copies are worth no more than $500.