Zoloft's peak plasma time is 4-8 hours, and a 26 hour half life. So if you blew off a day or two of no pill talking, AND are on a really low dose, it could happen .
As for things bipolar, you'll find out if you are while on Zoloft. Two weeks on it fueled my first mania and inpatient psych hospitalization.
For first time depression patients, Zoloft is never given because it can trigger rip roaming manias. My was leaving the country and refusing to come home. Good times, good times.