Code P0455 EVAP leak?

I was driving and while driving the check engine light came on but as it came on the rpm meter arrow dropped and went back up. The car felt like it didn’t want to accelerate or the acceleration slowed and the car sounded as though it wanted to shut off, but didn’t. I checked the light and it came back as a large evap leak but about 2 weeks prior I had the transmission fluid changed not all the way but partially the fluid was black and as far as I know the fluid is suppose to be clear or pink it was far from that. Any idea what I could be dealing with?