I'd take her to the doctor to rule out anything medical. It could be nerves, but there may be an underlying issue. The neighbor across the street had stomach pains, and would not go anywhere because it made her feel like she'd throw up (she never did). But this child did not come to play once last summer, for fear she would get sick. So definitely, take her to be checked.
I see you work full time, but are you able to have lunch at school with her? We are allowed to go for lunch at our schools. Could you find an hour to volunteer in her class?
When my 9 yr. old was in first grade she had a hard time adjusting to a whole day of school. We put a family photo in the front of her folder. So sounds as if you have done that, on her bookbag. I also sent a note everyday in her lunch (she never wanted to buy). On occasion if she had a hard morning leaving, I'd email her teacher and she'd tell her what mom said (just hi, or have a good day, etc).
Good luck to you, I hope you get to the bottom of this soon. But definitely get her to the doctor ASAP, poor kid has got to be embarrassed about having to go to the bathroom so much.
Does she have problems at home on the weekend too? If not, then it probably is nerves, or is her diet different on weekends as compared to weekdays??
Keep the teacher and the nurse informed, and tell your daughter if she has an accident the nurse is used to it and not to be worried about needing to go for a change of clothes.
Let us know what you find out.