SMS not sending to Samsung Galaxy S3

I have read a lot on the boards about previous iPhone users not being able to receive texts on their Galaxy, and issues with iMessage settings. However, I haven't found anyone with my particular problem.

My mom has never had anything apple. She currently has a Samsung Galaxy S3 and received my texts just fine. I have an iPhone 5 running iOS7. We are both on AT&T. My messages to her. go through as green, SMS messages. Always have and still are. However, all of a sudden she no longer receives my texts. I have deleted and re-added her to no avail. My sister has an iPhone 5C on AT&T and my mom still receives her messages, either direct or within a group text.

What could be the issue here? My mom brought her phone to the AT&T store and they simply deduced that it's something with my phone because she can send them to me. However, it almost feels like she may have blocked me. Unforunately we do not live in the same state and I can't work on it next to her.

I may try turning iMessage off altogehter but that seems a) irrelevant since my texts to her are green SMS and b) I don't want to not be able to use iMessage just because of one person who can't receive my texts.

Before iOS7 two of my friends with varying Samsung phones were unable to receive my texts. This was before iMessage, even. This is very frustrating, so I'm hoping for some great advice from the community!

Thank you! 🙂