Is it permissible to masturbate when one does not feel comfortable during intercourse with the husband, or when the husband refuses and there is no comfort after intercourse? And is there a sin upon her for that?

It is not permissible to practice masturbation, as it is forbidden. Its prohibition is even more severe for a married person. One should be open with their spouse to understand the reasons for the suffering. If there are any medical symptoms, a specialized female doctor should be consulted and treatment sought, while seeking Allah's help.