I think it is very sad that you feel guilty about asking them for something so simple when you have a perfectly valid reason for not having the dog in the house.
You will have had major surgery and won't be able to cope with the mess that the dog leaves.
Just because they have helped in the past doesn't mean you can never say anything to them that they won't like, or be firm about something you don't want to happen... You don't have to back date your requests! Situations change all the time and a new baby and elective cesarian are BIG changes and they ought to recognise that! Its not a lack of respect or gratitude and It doesn't mean you hate them or the dog. If they choose to take offense instead, you have to educate them.
They wanted to help you and you were grateful and thanked them. That is the end of the transaction. Otherwise it was conditional help with a price tag and the cost is now too high. Them helping you out doesn't mean you have to submerge your own wants and feelings about anything they impose on you from now on.
You are a mother and you have to stand up for your baby's needs and your needs. No one else will. It's not a question of guilt or gratitude, it's a simple question of you organising things in the best way for your child.
In my experience it is FAR FAR better to put up with someone being offended by a perfectly reasonable request (when the alternative is them continuing to impose their will on you) when it would be a simple thing for them to simply not do it. They have to start viewing you as an adult in charge of your own life and your baby's life and not as a child who can be ordered about.
So if you ask them nicely and they are offended, put up with it but stay firm. "I am sorry if you don't like my request but I have explained that I have very good reasons for making it and must stick to it" or something along those lines. Most people, when they see you are firm and not budging will gradually just accept it and find a work around - like asking a friend to check on the dog or meeting you at a park cafe or something, and then the problem is solved. You and your baby should be higher in the pecking order than a dog for heaven's sake!
If they are difficult about it, then there is a bigger problem to address, but it will show you that they care more about their own wants and needs than yours and you can start to dismiss the guilt for standing up for yourself. As a mother there will be plenty of occasions where you have to do this and practice makes perfect.
Sorry I don't mean to sound naggy, but have been in this situation with young babies and I do understand how it can make you feel awful, but it will be such a relief for you when you do manage to deal with it to your satisfaction.