Are my Apple Snails eggs fertilized?

I've had my Apple Snail in my tank for around 3wks and this afternoon i noticed there is a clutch of eggs on the underside of the lid! They can only of been there for 2 days max as i changed the lid on Thursday! My question is this, will they be fertilized eggs as she is the only snail in the tank?? Is there a chance i bought her already pregnant? How long is the gestation period??