She's 21 months or so? there's 2 possibilities:
1) She's still a bit OT from recovering from illness and getting teeth in so needs a slightly earlier nap (or to sleep a big longer in the morning, and then have same nap time); or
2) she's ready for a bit longer A time and so is UT at nap.
There's no way to know for sure except to use your mommy instincts or to just experiment! I don't think the waking after 30 minutes is SA. That's either UT/OT. At our house, 30 minute naps were always UT. But I know lots of people on here have 45 min UT naps. So, based on your experience pick which scenario you think is more likely and adjust nap time accordingly!