I would say this is unusual, at this age/grade.
HOWEVER.. it does happen, when the child is unhappy or stressed etc.
You need to pursue it further... to see WHY your daughter is having physical stress symptoms and emotional hardship about going to her class.
Is she being bullied/teased/picked on/ignored? ASK OTHER PARENTS from her classroom if their child "complains" about the Teacher/class etc.? Ask fellow classmates... how their Teacher is or what EXACTLY happens in class etc.
Does your daughter have a "reason" for "why" she is so unhappy? Because until now, she did enjoy school. So, to me, this is a red-flag of some kind.
IF the school-work is too hard as your daughter says.... BUT her Teacher says she is doing fine... Then, what is the contradiction here about?
Your daughter missing you all day, means to me that she is experiencing some kind of unhappiness or trouble or something is not, well. A child this age misses their parent when they are having problems or feeling at a loss for something.
SEE what is really bothering your daughter.
I would trust your daughter... and go according to her.
MAYBE, is she really ready for 2nd grade? Is she 6,7,or 8 years old? In my daughter's class, (she is also in 2nd grade), some kids are still 6 going on 7, some are already 7 going on 8, and some are already 8 years old. SO... there is a range.
I would also, make an appointment with the Teacher to discuss what is making your daughter so unhappy etc.
The previous respondent, Bea, had a good point.
All the best,