It very well could be the onset of puberty (it seems to start so young now). It could also be additional or increased stress. We don’t realize the impact of stress on brain function, but it can be very debilitating:
By this age there’s a lot more pressure to be “accountable” for oneself. And that’s often too high of an expectation for kids with ADHD.
Here’s some information on working memory deficits as well:
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism