Is she pregnant or not?

Recently my wife started to have some pregnancy symptoms. She missed her period which she says was suppose to have come on the 24th (so like 4 days late), shes been feeling very tire, been having a severe headaches for several days now, and shes been having some mild cramping. We both assume that she was pregnant but decided to take a pregnancy test to be sure. We took 2 first response pregnancy tests between a 8 hour period but they both were negative. Is there a chance that the tests could have been wrong? She has to works and she is also a part time student, could the stress be causing these effects her? I insist that she go to the doctors for a checkup but since we do not have insurance, she does that feel the need to do it. Could these problems a sign of a bigger problem/illness?