Talk to her. Explain to her that school can be hard sometimes and that you are worried that she may not be comfortable with the reading, spelling or other areas she is learning. Let her tell you her worries while talking about her day.
Meet with her teacher or daycare provider to find out the areas that she is getting frustrated with or doesn't like to do. Younger children haven't learned yet to hide things they are disappointed in; they might act out instead.
Notice if there are any problems with your kindergartener at home. Does she no longer want to read a bed time story? Does she resist when you ask her about school or what she learned that day?
Get a tutor immediately who has tutored kindergarteners before if you see any of the signs that your kindergartener needs some tutoring. It's best to get on top of the problem immediately before it is allowed to develop.