I don’t think Arbella suffered from porphyria. I think she suffered from long-term stress, disappointment and clinical depression.
Arbella was orphaned when she was very young. She was raised by her grandmother, the cold and calculating Bess of Hardwick. Bess encouraged Arbella to believe that she would inherit the throne of England, since the common law forbade foreigners like James from inheriting the throne. To freak James out, Queen Elizabeth maliciously encouraged Arbella’s pretensions at times, but did not officially claim her as her heir. As she came to adulthood, Arbella’s grandmother treated her like a prisoner and Elizabeth refused to allow her to marry.
When James became king, Arbella resigned herself to leading a private life and made no effort to plot against him. James never trusted her, however, and kept her under his eye as much as possible. Queen Elizabeth's mischief bore bitter fruit: James was seriously paranoid where Arbella was concerned. He too refused to allow Arbella to marry, despite the fact that she was eager to do so and clearly had no political aspirations.
She secretly married one of his courtiers and fled the country. With the French shore in sight, Arbella’s vessel was captured by English ships. She was brought home and imprisoned in the Tower. Her husband, who was aboard a different ship, managed to get away.
Since she hadn’t been charged with anything, Arbella dared to hope that James would let her go and permit her to join her husband in exile. Arbella spent her days writing letters and worrying, since she was running out of time if she was to have children. She pinned her hopes on the marriage of James's daughter, believing that once James secured his own succession he would release her. She even ordered an expensive dress to wear to the wedding.
When it finally dawned on her that James was never going to let her leave the Tower, Arbella had a nervous breakdown. Despite her harmlessness, she remained a prisoner until she died.