It may be be because you have set some limits and she's wondering if she can get milk elsewhere but it's ok to set limits and it's ok for her to discover for herself that only mummy has the milk.
Mother of two spectacular girls, born mid-2010 and late 2012
I enlisted a friend who understands enough about my family and our habits and culture, and is around us often, to help Little Miss understand booby boundries. Little Miss was mainly massaging other people's breasts, but it was "not OK" enough for me that I felt a need to end that behavior. So we did. No fuss, no muss, Little Miss is still a happy, loving child, just now she doesn't feel up grandmas in the grocery store. She still feels me up pretty regularly.
And on 09/23/2011, we were three; husband, daughter, and me!
Don't feel guilty about setting some limits on nursing. It's a two-way street and both parties need to feel good about the nursing relationship. This came up in another thread and a couple of moms posted about seeing other mammals putting off their offspring if they didn't feel like nursing. It's quite natural to want a little space after the many intense months of nursing an infant and a 19 month old has a lot more options for nutrient and comfort than at 19 week old. A few limits now may translate into extra weeks/months you're happy/willing to nurse later.