I think you are right — yes, they are
raising their expectations for you precisely
because you are a woman and more specifically
a mother. I think their motivations probably
vary — but it might be because they felt
unsupportive when they were mothers of young
children and thus they want to "retaliate" or
they might not be mothers of young children and
feel resentful that you are cut any slack and
therefore want to put up obstacles for you. They
perceive you as having it easier and thus want
to make it difficult.
I am not sure what state
you are in, but many have non-discrimination
clauses against breastfeeding women — but
it doesn't seem like your issues are legal as
they are emotional. I also think that your persevering
through this will likely make the situation better
for future mothers. You are likely paying the
price to enable them to have an easier time.
And that's probably not a huge motivating factor,
but to change the situation for yourself means
that others will benefit.