What did I do?

Yesterday morning, heading to work, only 2 blocks from my job, I noticed flashing lights in my rearview mirror. It was a police car. So I pulled over, assuming he was going to go around me and proceed to wherever he had to go. Unfortunately, I was who he was after. I was honestly perplexed and when he got to my window I asked what did I do? Apparently I went through a red light. He was a young officer, very polite. I got out my car wallet to give him my papers and felt like a confused old lady - I have a tendency to not throw away the expired cards when I put the new ones in, so I kept shuffling through three or four different cards struggling to find the current one. Luckily the officer found the right one. End of story, he told me to be careful and did not cite me. Lucky break. Vow to focus and pay more attention.