Since you have got three negative pregnancy tests
,the chances of pregnancy are very bleak.However you may get the blood HCG
test done to completely rule out pregnancy as sometimes there are very low levels of hormone in urine which may give negative result in spite of being pregnant.
symptoms are very common and generally the cause is psychological although sometimes physical causes like ectopic pregnancy,obesity or cancer can cause false pregnancy symptoms.
You must consult a Gynecologist and get properly examined and investigated for not only confirming the presence or absence of pregnancy but also looking for any physical cause behind pregnancy symptoms.