Meditation Experience

[LJ57]

This is not an experience that has anything to do with OBE but I thought maybe someone could shed some light on it. It happened about 4 or 5 years ago when I was meditating fairly regularly. One day I was meditating before I had to go back to school and I either fell asleep or went into trance. I'm not sure which really. But when I awoke from it(after about 20 minutes), I felt a really strange sense of peace like I had never experienced before. I was totally content, free from negative thought, and at peace. It was almost a feeling of nirvana. That feeling lasted for the rest of the day but diminished the next day and I was quickly back to my same old self(negative, anxious,skeptical, etc.) I've never been able to figure out what happened or why it happened. Nonetheless, I've never been able to experience that again despite further meditation practice. Can anyone shed any light on this? Thanks.