Suggest Treatment For Irregular Periods

Goodam, my gf and I had our first baby feb this year, she dint hav eperiods in march, but she had it in apr 4-8, then we had contact in apr 18, which is her fertile period i think, we re not trying to conceive though we used withdrawal method, she had her period in may 4-5 which lasts only 36 hours, (is this normal? she said it looks like real period with some clots and she changed her pads for 7-8 times) so to make sure, we took hpts may 2, may 10, twice may 14 and may 15 with hpt with 25mlU sensitivity, a friend told us we should consider ectopic pregnancy, then later that night she felt some pain and i rushed her to the hospital, preg test are done but negative, transvaginal u/s done but they say they didnt see anything, they say they did 7 dilutions but negative(i dont know this procedure) she is 27 and normal fit. we calculated her lmp (not including may 4-5, since it is not regular or maybe just implantation bleeding, could it be possible, we really dont know?) she is 6 weeks. Doctors says she is not pregnant (whew.....) Is there a possibility that she still might be? is it too early to test or I am just freaking paranoid.. Please help