When a pugs tail is straight down what does that mean?

When a pug's tail uncurls it means they are unhappy. My pug uncurls her tail when she is scared, or not feeling well or extremely tired. Just like when other dogs put their tail between their legs to show they are scared.

Our pug often straightens his tail - when he is asleep or relaxed, eating, or just 'moping around'. As soon as he becomes alert, wakes up, etc his tail instantly springs back up. As long as the tail returns to a curl when the pug becomes alert then it is just normal conduct.