If you see "Need Period Data" on your Apple Watch, but the data appears normal on your phone, this is likely a synching issue between the watch and the app. To try and fix this, double check that neither device is on "Do Not Disturb" mode. Close the app and reopen it or add something to your calendar. This should trigger a resync between the app and watch. Give it a few seconds to see if the resync happens.
Please refer to this Apple article for more help to resync.