Delusions and paranoia are not a given for all people diagnosed with bi-polar disorder. Additionally, there are many different kinds of bi-polar disorder, some are more high functioning than others.
http://www.mayoclinic.com/health/bipolar-disorder/DS00356
I have no idea what SG's medical history is/was, so I'm not inclined to jump to the conclusion that, simply because she had been diagnosed as having bi-polar disorder, that somehow must have contributed to her unusual behavior the night she went missing. A far more likely scenario, in my mind anyway, is that her strange behavior was due to intoxication.
1). We have a source close to the case stating that JB and SG left the house in a vehicle together for about 15 minutes. This could very well have been a drug run.
2). We have SG placing a call to a CVS pharmacy shortly before making a barrage of calls to her driver.
3). We have JB himself saying that SG went into the bathroom, and when she came out she was acting strange. I assume from this statement, that she hadn't been acting overly strange prior to that.
Now, both MP and JB have plenty of reason to lie about the events that transpired that night...prostitution and illicit drug use are both illegal activities. But there are some whopping grains of truth in their statements, imo. My personal feeling is that JB and SG did, in fact, go on a drug run and that this is what triggered SG's out of control behavior. Either JB slipped SG something, and she freaked out because she thought she had been drugged, or something she took voluntarily reacted badly with other medications she was taking. I believe that is what prompted her call to CVS. IF Shannan thought that JB (or some other party) had drugged her, her fear that someone was trying to kill her makes perfect sense to me.
JMO