When will my black molly give birth?

She's had a white ring around her anus for about a week, and she's very bloated and looks like she ate something twice her size, she's swimming up and down the planted corner of the tank, but stops and takes a break every minute or so to swim, hang around the filter/heater or pick at the gravel. Usually she interacts with the other fish but lately has been very detached. When do you think she'll give birth?