A.L.
It sounds like a whole bunch of things, all hitting at the same time.
Wrapping one's arms around a child "to calm her," may have felt like a threat. I would probably bite someone who grabbed me and wouldn't let go, too.
Allergy medicine does strange stuff to kids. Mine have to take it at night. They need it, they really do, but it makes them tired and very cranky, and more susceptible to upsets.
Plus, things have been changing, from her point of view, willy-nilly on her. Mom home, mom not home. Daycare one day/week, daycare full time. What has she had any control over in the last six weeks? Don't we all sort of lash out, when we are feeling lost?
At any rate, it sounds like you need to have a talk with the center.