Is my turtle depressed???

So…. I have three terrapins in a 90 Gallon tank– the water conditons are good and all seems to be well with my two larger 3 year old male and female Northerns — BUT… I have a little one &1/2 year old Texan that for the past two weeks has been acting very strange…. She has almost constantly been hanging out in this dark hiding area in the tank – and only come out to eat — She has been eating fine and acts normal otherwise– the two Northerns are a bit bigger than her but in no way have I ever seen them bully her- so I just cannot figure out whats wrong with her– she will still bask and seems to be healthy — any ideas??– is she depressed mabey?? — thanks Heather