The mechanism of St. John's Wort isn't known, but many speculate that it acts like a toned-down, classic antidepressant (like Prozac). While that doesn't scream "don't take this with estrogen," it does give you a hint that it probably isn't going to do you any good when used just when you're stressed out. Antidepressants take a prolonged, continuous use to work their magic. Additionally, St. John's Wort is known to change the rate at which your body metabolizes many, many drugs (changing how effective they are).
However, I'm not a doctor, and if you're concerned about it, you should ask one! Just remember that OTC doesn't always mean safe.