So under your mom's contact, there's no email address, and has her new phone number? What if you delete your message history (messaging --> messages in top left --> edit --> delete the last conversation you had with your mom) with her so that it doesn't automatically try to continue on that old thread?
That or try to manually select her from your contact list first (which should grab her number from there) and THEN from then on it'll assume you mean the number?
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