Weird waves of sadness

Does anyone else experience this? I’m 6 months postpartum and currently breastfeeding. I will go the whole day feeling good but as soon as I start to feed her I just get a big overwhelming feeling of sadness. It’s so strange because I think I’m fine but when I breastfeed, I get so sad and think so negative. Is this normal? Does anyone else feel this way? Is something wrong with me?