My sister in law

So my sister in law told me she was pregnant a few weeks ago. She showed me her 2 test & they were definately positive. Her other sister in law saw them as well. So to help her out i took her to a clinic where they do pregnancy tests for free...negative. we went to her regular OB/GYN clinic & same thing. Is this possible? Or could she still be pregnant. She is on Depo since July when she delivered her baby boy that is now 6 months.