Hi Mikka,
I also have BPD, diagnosed right shortly after I turned 18. I've been in therapy since I was 12. The BPD really began to rear its ugly head when I was around 14-15.
There has been a very big question surrounding my gender identity regarding whether or not I was actually transgender, because the events that caused my BPD and PTSD heavily surrounded my biological sex.
Like Amber said, BPD is often characterized by instability in most areas of life, including sense of self. It took 8 years from the time I was 10 for people to believe me that I'm actually trans, initially because I was "too young" and then because I was "too mentally ill."
But like everyone else has said, there is a disconnect between being trans and having BPD. While BPD can influence the way you view yourself and your gender in certain ways, if you have a consistent feeling about being genderfluid, then you're probably genderfluid.
With BPD it's also very difficult to pin down certain consistencies like that, and because of that it's likely you'll spend a while trying to figure this out, which is absolutely okay! In fact, I think taking some time to really notice it yourself is a good thing.
I wish you luck in figuring this out!