My 8 week old was exceptionally hard to wake up today. Anybody have this problem?

She was napping in her car seat when I noticed she hadn't fidgeted in a few minutes so I went to check on her. I couldn't tell if she was breathing so I stroked her cheek & got no response. My instincts went into overdrive, I started yelling her name while getting her out of the car seat - no response. I stood up & bounced her while singing and making noises and she opened her eyes, fussed a bit, but went almost immediately back to sleep! I was so terrified (thought I was witnessing SIDS) that I got her naked, and poured cold water on her tummy. She was then fully awake and bawling, thank goodness. After I got her calmed down, she was completely normal. She was alert, responsive, she ate like usual, she pooped, she even smiled at me! But, I've been up all night and I'm terrified that she'll stop breathing. Has anybody experienced something similar? Before you become alarmed: I called the pediatrician and she said that b/c the baby's color was good then she was breathing and doesn't need to go to the ER. Thank you!