maybe it's because after I completed the boss fight and then closed the game before doing the "sexy" or going to the other rooms
boss progress is saved after you defeat a boss, so you shouldn't have to complete the fight again, and item progress is saved when you pick it up, so if you did, you should be able to dash with "spacebar". If you can't dash, fast travel to the boss room (using the map "M" while in a pool), and the dash upgrade should be sitting there