Hi, I am a Japanese student and have a qustion about English expression.
In Rosamunde Pilcher's "The Shell Seekers," I came across the following phrase:

"A good time. Not a good TIME, but a GOOD time."

It is spoken by a female character when she tells her boyfriend that it's all over and that she is very grateful to him.
I cannot make out its meaning.
What is she expressing by it and what effect the emphasis gives?
Please give me an explanation about it.
Thank you.y