I'm installing a tub & shower combo in my basement. The 2" pipe after I uncapped it shows the pipe to Go straight down about 2' or so. I wanted to see how it drained because I could smell oder and decided to put some water in this pipe. The pipe filled up almost to the top and slowly went down so I'm lost and don't know where to go from here. Is their no p-trap, is it a venting problem Im confused.