If you follow the guidelines for personal bags on AA.com, you should be fine:
Personal item
Your personal item like a purse or small handbag must fit under the seat in front of you. Dimensions should not exceed 18 x 14 x 8 inches (45 x 35 x 20 cm).
I have a camera bag that is exactly those dimensions and have never had an issue putting it under the seat. Avoid bags that have rollers, as GAs may assume that a roller bag is beyond the dimensions of a personal item. Best to get a bag that you can wear on your back.