It would be impossible for us to tell you what's wrong, because we are not vets and we cannot diagnose your dog over the Internet. It could be so many different things. If your Chihuahua is having abdominal pain, then you need to get her to the vet and have her looked at today. Emergency Vets are open 24/7, so there are vets available anytime of any day of the week. She could have a blockage. If she does and you don't take her to the vet to get her checked out. If you don't, you can lose her. Please take her in! Hopefully it's something simple that can be easily treated.