Okay. That's weird.
I would be making a call to Mom and ask what's up. You don't know at this point whether she gave up your number, and if she didn't, maybe she'd like to know what her boyfriend is up to.
Future steppies? Really? After four months?
At this point, I think I would just call my mother and ignore the texts for now. I agree, it's inappropriate or too forced. It sounds like you all are still trying to digest the circumstances.
Keep in mind, everyone is going to be out of the house in four years or so, and your mom and dad deserve to have people to share their lives with while you're off living yours, so cut her some slack here. Still, this situation is rather odd. You already told your mother "not now," and this happens? Call your Mom. (and return and report)