I don't really know what the joke is here, but here are some ideas I came up with:
- April and Eva hate "Rebecca" so much that it makes them throw up.
- They really don't want to go on after "Rebecca", so between panels 3 and 4 they got wasted and now need to go to the bathroom.
- "Fluid" is actually Canadian slang for "angry", and they're headed off to the women's room to gossip.
- Eva and April are into watersports, and April finds being peed on so relaxing that she's asking Eva to do it in public.
- The fluidity they're referring to is actually gender fluidity, and they're headed off to the bathroom to get in drag in an attempt to upstage "Rebecca".