I need some help ladies my sister in law thinks shes pregnant. she has taken a pregnancy test and it came back negative,…

I need some help ladies my sister in law thinks shes pregnant. she has taken a pregnancy test and it came back negative, she also went to the health department and they gave her a pregnancy test to take and came back negative. they didn't bother taking blood test. she's freaking it out. she had boob pain for 3 months straight and her period hasn't came yet. we are convinced that she is. she was before in that past but she didn't know due to 15 pregnancy test coming back negative. she had a miscarriage... what's causing her to have a negative???