“Why does a very dutiful parent suffer from afflictions from his/her children? What is the cause of this suffering?”
I think the question carries within it a misunderstanding of the law of karma. The law very simply is “As you sow, so shall you reap.” If the law were indeed as mechanical as it is understood then it would mean that the Creator or the aspect of Him that renders Justice takes into account only the outward act and not the many ramifications and complex internal aspects that any action involves. Even a human judge does not render justice on the basis of the act alone but on the intent behind it. As far as the Divine Providence is concerned it takes into account not only the act and the intent (which we often hide behind acceptable terms) but also the state of consciousness, the level and stage of our development, the long chain of past that trails behind us and the inevitable future towards which we move. Even if we leave all this aside for a moment, logically the reward of goodness should be a growth in the power of goodness – nothing less, nothing more. Thus, for example, a moral man grows in moral strength by practicing certain qualities and virtues that he regards as sacred and his duty. It does not ensure him a smooth life or a return duty by his or her daughter or daughter-in-law. Such a law would be a closed system of justice that would not allow further scope of evolution. But the journey of life and the law of karma is, above all, about evolution rather than justice. Thus, for example, when a moral man is ready to evolve further, he or she would meet such circumstances in life that will compel her to turn towards God, no more for any earthly reward but for God Himself. It would be more helpful for such a person to be surrounded with people who are ungrateful, who give bitter experiences of life so that he/she can diminish the attachments and seek further. A dutiful daughter in law would mean earthly happiness that would tie her down to the same level of moral evolution. So, if you see it from the point of view of justice as secondary to the primary purpose of evolution then you would find this perfectly logical that a woman who has dutifully served everyone and taken care of everyone is not really cared for by her children. It simply means that her term of evolution as a sattwic person is nearing an end and therefore Nature and God want to take her to the next level, the evolution of the moral person into the spiritual one.
The problem is that most persons regard the moral as the ultimate step and hence wonder why when they are doing everything ‘right’ per the moral rule book of life, they may not be rewarded with all joys of earthly life. This may be an Islamic and Christian concept of God but not one of the Sanatana Hindu dharma which sees moral stage as a step in evolution, an indispensable step no doubt but not the last one. That is why there is inbuilt within the Hindu doctrine the idea of vanaprastha following upon grahastha, so that after a life well led according to dharma and moral principles, one is now qualified to go further by renouncing this stage willingly and go further in search of God, in search of the True, the Real, the Permanent rather than the apparent and the transient.
The pleasant and painful experiences of life are meant to lead us eventually to this evolution next rather than keep us tied into a never-ending circle of ignorance wherein we seek perishable goods and transient things forever, whether here or in some heaven. But most human beings do not wish to go farther, they are simply too happy to be in their comfort zones even though they suffer. Hence the old age, devoid of real meaning and a lasting true purpose is full of misery of various kinds until death gives the final blow and then one has to go through the same rigmarole in lives after lives. But the wise soul learns fast and renounces willingly what in any case will be snatched away one day by Time. While still in youth such a person strives for the True rather than the apparent, the Permanent rather than the transient. Those who do so find their load of miseries vanish. They who don’t, spend their final phase of life full of endless cursing and complaints, mistaking a Grace for an affliction and God’s Love as His punishments. This Ignorance is the source of all suffering, but men love it and do not wish to be free from it until a sharp pain and a blow strikes their life! And then we wince and say how unfair and unjust You are because You snatched away my toy and did not allow me to play forever in the kindergarten of life!