Help with 9 month old schedule. Regression?

My 9 and a half month old has been sleep trained since 6 months, we used CIO method and after about a week was sleeping 11-12 hours at night. She would have one wake up a night on some days around 3-4am and would nurse for 15 or so min and go back to sleep.

The past few nights she has been waking up around midnight which to me seems too early to be hungry? She just sits up or stands up in her crib and cries. I nurse her back to sleep since it’s the only way she’ll fall back asleep. She hasn’t fully slept through the night in a while. Could this be a schedule issue?

Her wake windows are usually around 3/3/4 with 2 naps that are anywhere from 30 min to 1 hour. She has always been a very short napper no matter what we try.

Could this be a schedule issue or the 9 month regression? She was also sick last week so we let her sleep a bit extra then with the holidays bedtimes were pushed a few times but for the last 5 days she’s been back on her schedule and still having this weird midnight wake up.