According to press releases issued by the Trump campaign, Palin was supposed to spend today (and the next several days) criss-crossing Iowa with Donald Trump, stumping for him. However, at the rally Trump held this morning, Palin was absent with no explanation from the Trump camp. Instead, Trump just took the stage for a little longer than planned.
The media inquired about Palin's absence and were basically given "no comment" in response.
Speculation, of course, is that Palin has gone back home to help address the legal ramifications involved with her son's latest arrest. However, the arrest had already happened the day before yesterday and presumably Palin missed his arraignment yesterday in the course of giving her endorsement speech for Trump, so it seems that she made the conscious decision to keep with her political schedule yesterday.
It's equally possible, of course, that the Trump camp has politely asked Palin to step aside, as her speech yesterday was widely panned both in the conservative media and also by the audience who attended the rally.
At this point, in other words, it's still unclear whether Palin's unexpected absence from the campaign trail is due to her decision or Trump's.