My BPD and BP definitly feed off eachother.
Its not always as simple as pinpointing what caused which reaction and dealing with the outcome. Sometimes things snowball and get way more complicated than "normal"...
For example, my BPD could be behaving, and by that I mean I manage to not indulge in the behaviours and thoughts my sick mind wants me to, but then a BP episode hits and its like borderline on steroids plus bipolar!

all that resolve and self-leashing just goes down the toilet, and what's horrible is that there's an even bigger mess to clean up and bounce back from.
My bpd stuff will clear up relatively quickly once the trigger has been addressed, but if there's a bp episode present simultaneously then it takes about 2 weeks before the bpd stuff just magically disappears and becomes a non issue.
Idk if I'm making much sense, I'm not very articulate atm as I haven't been getting proper sleep for like 3 weeks and I'm close to crying from exhaustion... Just wanted you to know that I get it, and I know you're very well aquainted with you bpd so I believe you when you say that there's something more going on.