Back to work, pumping, and baby not nursing much

I went back to work 3 weeks ago. My LO is almost 6 months and except for about the first 3 weeks after she was born, she has been exclusively BF. I nurse her in the morning before I leave, at night before she goes to bed, and at least twice in the night when she wakes up. I have been pumping at work, but production has been inconsistent, so I pump at home once or twice to help get enough to keep up with her consumption, which recently increased to 4 oz, 3x per day, with the nanny. I nurse all day on weekends. We struggled to get her on a bottle just before I went to work, but now she's great at it. Only problem is she seems to not like to nurse as much - she will only go for about 4 minutes, and then she pops off, has to burp, then refuses to latch back on. It started off with her doing this just in the morning feedings, but now it's pretty much every time I nurse her. So is this just a phase, or is she weaning herself? It's making me sad, I really was hoping to BF for a whole year, I love the intimacy of it, and especially the health benefits, but I'm not going to be able to produce enough to do that if she won't nurse when I am at home. Any others with this experience, and any thoughts or advice?