Obviously I don’t have the entire picture, and as such can only speculate.
If I were to do that, I’d say that your mother has conflicting thoughts and feelings. On the one hand she cares for you, wants to be supportive and knows that this is your own decision - on the other, she dislikes and has a hard time accepting it. For the sake of the former, she tries to put her opinions behind her, but sometimes fails to do so (perhaps tied to mood?).
I have a similar situation with my mom, but instead regarding choice of career. It feels hurtful, and might never go away entirely. How you deal with that is up to you.