A.C.
Deb J. has some good ideas...
I realize this upset you but I would try to find a way to laugh this off to yourself or laugh about it with your husband privately so you can internalize it a little easier on the heart--He loves you dearly no matter what name he is calling you. And you know he's testing you b/c that is the age and there are more tests to come.
What do your stepchildren call you? Maybe your toddler just hears you being called something different by them whereas everyone calls your husband "daddy" so he is a little confused? Can your husband refer to you more as "mommy" in front of your toddler for a while and see if this influences him? If you have a small photo album with pictures in it have DADDY go over them with him and insist he uses the term "mommy" at your picture.
My toddler has responded really well to my relationship with my mom and mimics me talking to my mom using the term "Mom" affectionately toward me. So if that is an option then a little extra exposure to you and grandma could help too.