Did I commit a major sin by intentionally breaking my fast based on the expectation of menstruation, and does this entail expiation by fasting for two consecutive months?

You must make up for the days you missed fasting. No expiation (kaffarah) is required of you, because expiation is only obligatory for someone who deliberately violates the sanctity of the month by engaging in sexual intercourse. We hope there is no sin upon you for deliberately breaking the fast, as you did not do so intentionally. However, you must seek forgiveness for your negligence in inquiring about and learning the obligatory rulings.