what was the source of mrs. clutters illness?

Yes, chronic postpartum depression

In regard to his family, Mr. Clutter had just one serious cause for disquiet--his wife's health. She was "nervous," she suffered "little spells"--such were the sheltering expressions used by those close to her. Not that the truth concerning "poor Bonnie's afflictions" was in the least a secret; everyone knew she had been an on-and-off psychiatric patient the last half-dozen years. Yet even upon this shadowed terrain sunlight had very lately sparkled. The past Wednesday, returning from two weeks of treatment at the Wesley Medical Center in Wichita, her customary place of retirement, Mrs. Clutter had brought scarcely credible tidings to tell her husband; with joy she informed him that the source of her misery, so medical opinion had at last decreed, was not in her head but in her spine--it was physical, a matter of misplaced vertebrae.