5 month old puppy cries when she's put out at night?

We've had her since she was about 7 weeks old and had the usual puppy trouble for the first week or so.. But now she is a few months older she sleeps outside at night and until very recently, has went out at night and gone straight into the kennel and that was her till the morning. However, for the past week she has been very unsettled at night and cries/barks for hours, no matter what we do. Our routines have not changed as far as i know, she gets a long walk each day and plenty of attention/playtime during the day... Any ideas?