You turn off the shower manage to pull yourself over the side of the tub onto the bathroom floor. Fortunately your bath towel is close by and you are able to reach it from the floor. You began drying yourself off, starting at your neck and working your way down. As much as you enjoy being a mermaid, you need to get to school. You'll get to see Amy there and you think fondly of your time together yesterday. Her touch, her caressing your body, her tongue...
You shake your head. You need to focus. You towel your mermaid tail dry, down to the tips of your red fins.
You stare at your tail in confusion. You've dried off but you haven't returned to human form. You think back to your time with Amy. She turned into a mermaid in her pool, she woke up legs in the morning. She turned into a mermaid again in her shower, but dried off and got legs again. Then the bathtub where you both became mermaids then both returned to normal after you got out of the tub and were...mmmmm... You're again overwhelmed with memories of the previous day. Your hand slips down to your crotch, feeling your wet lower lips.
There are a couple possible explanations you decide. You might not be able to transform back while aroused, or maybe not without Amy's participation. This is a weird side effect of her desires after all.