I was on exactly the same medicine, and I firmly believe that it helped me get to 36 weeks with my second pregnancy. The reason, I think, that it's category D, is because atenolol can make smaller babies in women who take it. For that reason, they'll probably watch your baby's growth very closely by ultrasound, and if there's any slowing, they'll probably adjust your dosage.
No doubt, it's the sort of medicine that should be taken only by people that clearly need it- for some of us, we really need our medicines to work. There are some more commonly used antihypertensives during pregnancy, but many of them are not really used for non-pregnant people, a testament to how well they work.
And, from what I've read, given the protein you're showing, they're doing exactly what they should be! Please let us know what they find!! Good luck!
Allie 5-13-98 (35 weeks-pre-e)
Baby Camille 4-17-03 (36 weeks- htn and oligo)