I have this same issue. Noticed with one iphone user a few weeks ago, then with my brother a few days ago, and today at work.
however, my number hasnt changed in the past 4 years, and i switched from Iphone to Htc One.
Its weird tho, the guy texting me today responded to a msg i had sent, but i didnt receive his msg. we checked it out, n the text he sent me was an imsg, however, a few days prior we had exchanged texts no problem. Those texts sent were not sent as imsg. Not sure why but the iphone seems to pick n choose whether to send regular sms or imsg..... i'm just realizing how many potential msgs i've missed in the past two months...