Background Pony #4884

She most likely smelled Starlight's Scented Shampoo that got washed in her mane earlier and the scent is making her horny, hence the 'wing boner' but she's keeping her other wing down so Starlight won't notice what's going on.Twilight:'What's this? Is Starlight wearing Scented Shampoo?'Holy Celestia, that smells really good!'Twilight:'Damn it.'