It's a strong possibility you just woke up right after a REM cycle, or right before a deeper stage of sleep. About 4.5 hours into sleep the average person is in stage 3 sleep (stage 1 is REM/light sleep) so that lines up with your story. Another possibility is that that night just wasn't the night. That happens to me occasionally when I try and WILD. I'll set it all up, be sure I set my alarm for the right time, and then after I do my WBTB and everything, and I just can't fall asleep for whatever reason, or I just fall asleep too fast. No harm either way, but if you try again for the same time and it happens again, you could try moving it back 20-30 minutes to try an catch that REM cycle better.