Reality Shift

Calvin Duffy thought his life was normal until it was not. The abnormalities begin as minor things suddenly changing— the color of a familiar wall, a highway exit he has driven past a hundred times, the names of children in his neighborhood — but none of these changes seem to register with anyone else. To them, things are the way they've always been.
The abnormalities gradually get worse. Someone from his office building vanishes as if he'd been erased from the world and no one notices or cares.
But Calvin does.
A woman shows up on his doorstep, claiming to be a family member, and she is, somehow, though she never existed before that day, before that moment.
Calvin is worried and realizes that he can't continue to ignore the problems but has no idea what's causing them or how to make them stop. Turbo, Calvin's mentally ill homeless friend, says they are happening because Calvin is shifting into new realities, one after the other, each slightly different from the one before it, in one long chain of parallel universes. The differences between these realities are small at first but grow larger with each new one that he enters, and he struggles to adapt but never really has time to before he's thrown into another.
When the situation becomes so bad that people close to him are threatened, Calvin knows he has to do something soon, and if he doesn't, he could end up in a reality where he loses everything.