As the daughter of Cato, a noble-born, Portia is an intelligent woman who demands to be an equal partner with her husband Brutus. Her argument, that she is not an ordinary woman, since Brutus chose her, in conjunction with who are family is, gives her the right to know what is going on in Brutus's life outside the home. She wants to share his life completely, as a wife. She sets out to prove to Brutus that she can handle the burden of knowing all of his secrets.
She makes it clear to her husband, that she is physically strong, to express this concept, she stabs herself in the thigh, to prove to Brutus that she is capable of bearing pain.
"Her anxiety over Brutus's mission, she also considers a weakness, as though love were a weakness."
"Her final proof of strength comes when she horribly kills herself because of Anthony's growing power which is a threat to her husband."
But once Brutus unburdens himself and tells her about the secret plan to overthrow Caesar, she cannot talk to anyone about it because she is a woman. She must keep silent. Once Brutus leaves, she is frustrated by the need to keep silent. Her horrible death, swallowing burning coals until she suffocated, is both symbolic and courageous.
Symbolically, she silences herself for good, with a method of death that obviously was excruciatingly painful.