We removed our old tub/shower and put in a new shower stall only. We moved the shower to the opposite wall and had to move some plumbing and the sewer hook up. The couple times we used it it worked fine. Now it drains very very slowly. This is what we did. We moved the shower drain approximately 3 feet, installed 2 in. ABS drain and p-trap with a 1/4 -3/8 in. drop per foot into the existing plumbing. We tied into the existing vent which was there for the tub. Do I need a vent closer to the shower or before the shower?