It sounds like she's using you as a dummy to me! My son would quite happily lie sucking on the breast for hours on end just for the comfort. I eventually learned when he was effectively feeding and when he was just having a token suck from time to time - it's not easy to tell the difference! He wouldn't take a dummy (have you offered one? Worth a try!), so we had to just find other ways to comfort him. Generally when daddy was holding him he was more settled as dad didn't smell of milk.
You should try to discourage the comfort-sucking if you can as you're more likely to end up with sore nipples in addition to everything else. If she's doing lots of pooing and your health visitor is happy with her weight gain then I'm sure you're producing plenty so try to have faith in your boobies (easier said than done I know!)
Well done you, breastfeeding isn't always as easy as it looks and it sounds like you're doing a great job. Should you need reassurance/advice, I can personally recommend the nct breastfeeding helpline - I called them twice and spoke to lovely ladies who didn't make me feel silly!
Good luck x