transmission issue

i recently got this 2000 isuzu rodeo, the transmission is slipping in the lower gears, i assumed it was low on fluid but after warming it up parking on flat ground and removing the fill plut it poured out like i had removed the drain plug and now it is slipping worse im not really sure why it would be running out like that as there is no way they added that much fluid without it running back out while putting it in any advice would be helpful i have never had an isuzu until now so i know nothing about them