This play deals largely with the pastoral, which in Shakespeare's time was idealized as perfect, carefree, and an ideal way of life. However, the pastoral depicted in this play has a darker side. Not everything is "ideal" in the lives of the characters...for instance, a terrible storm threatens the safe arrival in Bohemia, once there, Antigonus is chased and eaten by a bear, and there are angry outbursts from characters which indicates that not all is well in paradise.
Perhaps we can take from this "other side of the coin" interpretation of pastoral life that the title comes to indicate that there are opposites to everything. Pastoral life is usually portrayed in the spring and summer months where people are more carefree and joyful...the weather is warm, love is in the air, food is plentiful, birds are singing, blah, blah, blah. The opposite, however, is true in winter. The cold puts a damper on people's mood, food is not growing--and, unless people worked hard and stored food for the winter months, it is not plentiful--birds have flown south, etc.
The title, then, may just be showing that to everything and everyone there is an opposite, another side, a different mood, a darker more foreboding perspective.
Hope this helps! Good Luck