Controlling more holdings than your demesne limit allows gives you significant opinion, income, and levy penalties. These penalties do not affect you for a few months- giving you a chance to give away conquered lands before suffering penalties.
You were over your limit (well over your limit) for a few months and the levy penalties kicked in. To solve this, give away titles (or increase your demesne limit) so you are no longer above your demesne limit.
You won't get personal levies from the holdings you give away but you'll get more in total (and get vassal levies from the ones you give away).