Not every person with BPD is unfaithful. She may not be. Unless you have any way of knowing, do not accuse her.

It's just like not everyone with schizophrenia goes and shoots up a public building. Just because some do, doesn't mean everyone with that disease does.

If you are truly concerned, GET HER SOME HELP, and THEN, get tested for STD's and maybe start using condoms from now on.