Q: iMessage sending as SMS

I have an iPhone 6S Plus and my iMessage messages aren't sending as iMessage. They're sending as regular SMS messages even though I have the setting "Send as SMS" off and I have full 4G LTE signal. This was even happening at home when I had a good WiFi connection. Another thing I noticed is that this only happens when texting my girlfriend. When I text someone else with an iPhone, the message is blue indicating iMessage. She has an iPhone 6S and we share the same Apple ID but we both have the "send and receive" setting set to our own, she has hers set to her number and I have mine set to mine. So before this, everything was fine. But this morning is when I noticed this happening. I also got a notification that said something like "Your phone number is now being used on Kaylie's iPhone" or something like that and I think it said something about the Apple ID too. I don't know if that notification is significant to my problem. I already turned off iMessage and turned it back on and still no cigar. I just want the messages to be sent as iMessage again. Thank you in advance