A lot of AD meds, when given to someone with bipolar, can induce mania, so many physicians are very hesitant to use ADs, and when they do, the dosage has to be very carefully monitored.

Did your doctor increase all your other meds and leave the Lexapro, or did you choose to increase them? Either way, your doctor needs to know that you've increased them (either arbitrarily or as directed) and your mood is tanking. They'll need to make some adjustments to help you through this.