Were you traveling with someone and if your seat neighbor also had those bites?

Happened to me twice before (not on AA), once in Iberia economy and then once in KLM business. In the KLM business class case I went onboard fine, took a shower before leaving the house, and when I arrived Amsterdam Schiphol, in the shower room I noticed a bit of itch and then many bite marks on my back. So that was most likely bedbugs in the KLM cabin. You should report it, maybe send the complaint online, AA might throw some miles your way.