could it be possible to get bipolar disorder and pre menstrual syndrome confused?

Dear Ms. C' -- 
Ah, a most excellent question.  Do you know, I have not seen this question asked or wondered about by a psychiatrist yet?  And yet, from my experience with patients, it is a very important question.  Wondering about this led me to try to put together a website on "Hormones and Mood", hoping that lots of new thinking and research was likely to be pouring in and might need a forum for presenting it to women (and their male partners, too, one might suppose).  And there is a dramatic increase in psychiatrists (many of them women) starting to write and research in what I've heard dubbed "Gynechiatry"!  Anyway, you can read about my perspective on this question on the Hormones and Mood site, particularly the picture in the introductory essay.  

A brief answer to your question would be: "yes".  But it's probably much more complicated than that.  The symptoms look extremely similar to me.  There might even be a version of "bipolar II" that is the women's version, which usually appears in late 30's and early 40's and seems to be associated with having had at least one child (I seem to see the serious versions begin after the birth of a second child, although I've never seen anyone else speculate about this)  (let me know if you know of any such thinking, ok?).  At minimum, women with clear-cut family histories of overt bipolar I -- i.e. women whose symptoms are very likely to have something to do with bipolar disorder, not just some hormonal variations -- often have a return of symptoms they used to have all the time, just before menses.  It's as though the "last thing to go" as they get better from clear bipolar mood swings, is the premenstrual increase in symptoms.  

Dr. Phelps