The story is related in the most detail by Plutarch, who says he doesn't believe it, at least not the sequence of events. Cassius Dio just laconically says she swallowed a coal. Appian says that she reached out, although she was being watched by her attendants, and grabbed a hot coal as soon as she hears the news of the two's deaths. Valeria Maximus tells a somewhat romanticized version of the same story.

The other three give versions that Porcia killed herself after she heard Brutus and Marcus Cato were killed. Plutarch does not. Citing Nicolaus and Valerius Maximus he gives the story that she snatched hot coals while her friends and attendants were watching over her to make sure she didn't hurt herself (presumably they didn't allow her any weapons), but says that they must be mistaken. Plutarch says that there was an extant letter of Brutus' in his time that chided his friends for not watching over Porcia while she was sick--if Plutarch is right, Porcia could not have died after Brutus. This seems likely--Cicero wrote a letter to Brutus that survives in which he tries to comfort him regarding Porcia's death. Brutus also wrote a surviving letter to Atticus in which Porcia's ill health is mentioned. This throws something of an issue at us, since Porcia's death by coal-swallowing after Brutus' death is well-attested, or rather there was a strong later tradition--Martial even wrote an epigram about it. If the letter Plutarch refers to existed (and he expresses some doubt that it is necessarily genuine) then it would fit in nicely with what we know from Cicero. As it is, Cicero doesn't actually tell us what she died of--her sickness is only incidentally to her death, it's never actually said to be the cause of death.

Porcia's character certainly might have spurred her to such action. As Plutarch himself admits, she was devoted to Brutus and Plutarch says that Brutus' letter says she was so sick that she preferred death to life--whether that means she killed herself born just wanted to die is a bit unclear I think. If Plutarch isn't joshing us she was in a distressed state of mind--he says that she was so grieved when Brutus left for Greece that she cried out, quoting part of Andromache's speech to Hector, the bit where Andromache says that Hector is her whole family (Andromache's parents and brothers were all killed). Plutarch says that when Caesar was murdered she was in a frenzy--he compares her to a Bacchant--and panicked, searching around for any news of Brutus' safety, and ended up working herself up so much that she fainted. When Brutus was deciding whether to join the conspirators (again, according to Plutarch) he stopped talking to her--seeing that he was distressed Porcia resolved to ask him what was wrong, but only after she submitted herself to a test, stabbing herself with a knife in the leg. She suffered chills and fever, but summoned Brutus and showed him that it she was resolved to stand beside him no matter what troubles came--she was immune even to pain. So if this is an accurate assessment of her character she was certainly able to do something like that.

I think the verdict's still out on this one. Normally I'd say that when so many sources agree on something there's no good reason to doubt that it's true--but Plutarch's own doubt and testimony about Brutus' letter, as well as Cicero's letters and Brutus' surviving letter to Atticus complicate things. They do not necessarily contradict them, however--Porcia might have killed herself after false news of Brutus' death was brought, after First Philippi when he was thought by Cassius' force to be dead. As for the method of her death, that's really not resolvable I think. The coals story is pretty well-attested, and surely if she killed herself she must've done so in a somewhat desperate fashion--the repeated attestations that there were attendants in the room prevents her from poisoning herself or something like that. An interesting note is that Cicero's letters to Brutus is usually dated as late June or July 43, before Philippi. If that's genuine then Porcia's death can't possibly have been because of Philippi. I personally see no reason to doubt Cicero here, but even if her death occurred a year before Philippi, that still doesn't entirely rule out the possibility that she killed herself--and if she did so it may well have been with coals. The letter Plutarch refers to apparently says that her illness made her prefer death to life--what does that mean? I think it's possible to reconcile the two traditions--indeed, Plutarch doesn't question that she killed herself or that she did it with coals, he questions Nicolaus' timeline, sauing she couldn't possibly have died when he says she did