Seperation Anxiety in my Dog...

I hope you can help me. I have an eighteen month old Border Terrier called Lucy. She has been housetrained and was doing fine. But recently she seems to have regressed into a state where she suffers from separation anxiety quite severely. I’ve tried training her again to show her that whining and crying won’t have any effect and I’ve also tried giving her a treat before I go out and when I get back, but she appears to be in the same state of distress when I get back as she was in as I left. I can’t think of anything that could have triggered...