8 mo. old baby not interested in feeding...?

My girl is a little over 8 mo. and in the last week it has been increasingly more difficult to nurse her. She has always been very distracted and we got into a pattern of nursing laying down on a bed which worked very well for over a month and now she just pushes me away and cries. I've been pumping and giving her milk in a cup with a straw which she literally chugs but she just won't nurse. The only successful nursing session we have is before bed. She has begun teething in the last couple weeks but it doesn't really seem to bother her except for some mild fussiness and night waking every few days. Should I just keep pumping and using a cup? It seems I can't pump enough milk and I'm concerned she's not getting enough liquid nutrition. Any suggestions from other nursing moms? Is it possible she's weaning herself? She eats really well and has 3 meals a day and seems very happy but I'm getting so frustrated and it's awful! Just looking to hear what others think and any similar stories. Thanks!