iMessage vs SMS on command
I would check your address book entry on your mothers iPhone. iMessages can be registered to phone numbers, or email addresses. My guess is that because you're sending iMessages they are arriving under your email address on your mothers iPhone. Then, when she sends SMSs to you they go out under your phone number. So, check her iPhone's address book. Ensure that your entry has both your email address and phone number under the same entry.