To me it looks like she tried to talk to the ex, but was frustrated because he couldn't be reached, so she went to visit him. It's also possible that she was affected by the argument at the bar (where she was denied more drink) and/or by the road rage incident she was involved in shortly after the bar (where she pulled a gun on another car with three young women). Her last three calls from the ex's building (at around 6pm) to three different people didn't reveal the motive. There are no details known about the content of the call caught on video in front of the elevator, only that it didn't provide any insight. On the second call Prisma was frantic and allegedly mistook the person for her ex and on the last call she seemed confused, not knowing where she was.
It's not clear if Prisma and the ex broke up (again) that afternoon at the bar. IIRC he called her once more after he left. We also don't know why she stayed at the bar alone for hours. What was her plan? She didn't intend to return to work.