My friend is mad at me. What do I do?

She is my best friend and I was upset today and exclaimed how she sometimes acted like she didn't care and that our relationship seemed one sided to which she replied "If it's so one sided, then stop talking to me!". Realizing I had made a mistake and not wanting to lose her, I quickly apologized. She told me we'd talk later and we did. When we did, I apologized again and she forgave me but she said we shouldn't talk for a few days because we both needed to cool off. She also said she needed time to think things over. What does she mean? Did I screw up majorly? I love her tremendously and don't want to lose her.