BP controlled but protein increasing?

Hoping someone can help who understands how hypertension/P.E. works... My BP has been stable for my whole pregnancy and then at my 36 week appt, had gone up. A whole bunch of urine and blood tests came back ok, and my OB put me on labetalol 3 times a day to keep the BP down. I'm checking BP at home myself 3 times a day between OB appts and using my wee sticks to check for protein daily.
My protein sticks are starting to show positive but I'm confused because my BP has been under the 140/90 almost the whole time I've been medicated?
If the meds are keeping the BP down (still above my normal, but down) then why would there be protein increasing? I was of the understanding that high BP caused the protein and it really hasn't been very high since I have been on the meds.
I'm so confused and frustrated that I don't understand how this works :( I thought this would stop it progressing. OB his told me to call when the pee sticks change but he's not in his office today and I feel like a PITA calling the hospital.
Has this happened to anyone else? Or can anyone tell me why?