Hi! I went through the same situation with my daughter when she was this same age(the last 6 weeks of school). She would actually start worrying and crying late each afternoon and beg not to have to go to school the next day. It was a very difficult time. We reasoned with her the best that we could, but didn't give in and let her stay home. We finished out the school year like that. The following year things were fine.
My daughter is grown now, but occasionally she would have bouts with separation anxiety from time to time. Usually this would happen after something unsettling happened in her surroundings.(a family crisis, a teacher that wasn't especially sympathetic to her needs, an embarrassing incident, etc.) Think hard and see if you can recall something that may have happened recently in your child's life that may have affected her--it may be something quite simple, but unsettling to her.
I also worked in first grade as a parapro for 15 years and during those years we occasionally would have a student that would get separation anxiety and cry at school. Sometimes it would go on for weeks. I was always extra sympathetic with the child and would give them extra attention during the crisis. Usually through talking with them I could figure out what had unsettled them, just by listening to what they said.
Hang in there--it will pass. And know that you aren't alone!