Dargumin 
However to me, she appears to have a strong sense of self, she doesn't seem to be changing her interests & beliefs from bf to bf.
It seems sometimes that someone has a strong sense of self when they take a strong stand on a particular interest, want, or opinion. So it's possible to have a very poor and unconsolidated sense of sense while still retaining very strong opinions about things. For example, an NPD is (also) thought to have a fragmented self, but may have that characteristic unshakeable opinion on their self-importance.
E.g., your ex similar to my ex had a belief that if her partner didn't share a taste in x music and y drinking style, then that person "was not meant for her".
So with this in mind could she still have BPD? or more likely something that may appear similar such as PTSD from abandonment / relationship anxiety maybe?
For BPD I think the suggestions from
Purplex and
Turkish--to recognise that the sense of self is one of nine criteria--is a very helpful answer. For the second part of your question it might help to get a T's opinion as a supplement to answers you get here--plainly because each of these issues can get very complex. So thinking forward--if it's BPD, PTSD, or another anxiety-related disorder--does this have a consequence on your relationship life?