girlfriends period came when expected but...

Texas

so we had a pregnancy scare her first period was 1 week late but a but consisted of moderate bleeding then it tappered off. Her second period started Wednesday and the flow was real heavy then thrusday it went to moderate flow and today only the pad gets blood every few hours with wipes yeilding small amounts. Could she be pregnant? We took multiple pregnancy test over the course of 4 weeks and all came back negative.