Please help! What Could Cause False +? Anyone had this happen?

Sarah

My bf has not had a period yet after her baby was born. So she can’t track her cycles yet. But she’s been feeling nauseated and tired and decided to test. She keeps getting lines.... but then she will also get negative results as well. What in the world is happening? Has anyone had this happen? It’s just baffling her. Any thoughts welcome!!! Please help!