Have you spoken to your child's teacher about your concerns? Did your daughter ever take any standardized math tests (a test that the whole grade would take at the same time)? Is there a guidance counselor or person in charge of testing you could talk to at the school? Is there any specialized math help available for children in school who need it?
Testing would certainly give you more information about what's going on and you have the right, as a parent, to have your daughter evaluated at your local school or school district. But testing takes time and it will be a while before you get the results. In the meantime, it might be a good idea to consider having her tutored by a professional who is skilled in this area. It sounds like there might be some tension building up between your daughter and your husband when he helps her with her schoolwork. It's often a relief to all concerned to have someone from outside the family do the tutoring.
Your school guidance counselor may be able to give you a referral or you might consider looking at the websites for either the Learning Disabilities Association or the International Dyslexia Association to see if there's a branch of either organization in your area. Both organizations should be able to help you find a capable professional who can help with math. Your local college or university may also be able to recommend someone who has been trained to do this work.
You're very wise to take action now before this difficulty she's having affects her self-esteem further.