The cause for irregular periods
can be either pathological or hormonal.Since blood tests have ruled out thyroid and pregnancy
,then its possible that amenorrhea
can be due to pathological cause.First i think you need to rule that out by a ultrasound
.Iff its normal,then the amenorrhea can related to the hormones.Since pregnancy is ruled out,just wait for a few more weeks to see if your period comes.If not,you will have to take medicines for withdrawal bleeding.Also if you are breast feeding,there may be some amount of amenorrhea.