Well if it's true that her breakdown was due to a personal matter she could and should have excused herself from PMQs on account of being unwell or dealing with a family crisis and no-one would have been any the wiser. Obviously the timing of it would have caused some speculation on the back of the humiliating climbdown she's just had to do (again) but it can't possibly have been any worse than the speculation this public weeping and loss of gravitas is causing.
If it was because of having the mother of all shit days at work, feeling unsupported by your boss, thrown under a bus by your party and hated by the country, then as Chancellor of the Exchequer sitting weeping on the front bench, of course she's going to insist it was due to an unrelated personal matter.
This is what politicians do. Lie to us when it's blatantly obvious they are lying, pretend everything is fine and that there are no behind the scenes dramas or fall-outs when it's obvious there are, and spin shit situations into something they think sounds plausible.