While May was not explicitly shown to have a pre-existing heart condition, the events leading up to her death suggest that she may have been prone to heart problems due to her stress and emotional trauma from the past.
It's important to remember that May was a deeply traumatized woman. She had lost her daughter, Lily, to an illness, and she was still haunted by the events of her childhood. These events caused her immense pain and grief, and it's possible that this emotional stress contributed to her heart attack.