Snape very probably didn't have a Patronus before he turned back to Dumbledore. JKR said most Death Eaters didn't or couldn't cast a Patronus. Snape could only cast his because his love for Lily was the only good thing in his life. Therfore his didn't change.
Well, that was their Patronus before they were married. A Patronus can change it's form. Snape is a great example, but I don't know if their Patronus changed when they got married. I'm going to say that they don't have to be the same, though.