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The symptoms that you are having are most likely due to hormonal disturbances that ladies can have quite commonly.
This can even happen due to excessive stress, either physical or emotional.
It can happen due to a variety of other causes such as deranged thyroid hormone levels, deranged prolactin hormone levels, pregnancy (which has been ruled out in your case), to name a few common causes.
If the irregularity in your periods persist I would recommend that you should contact your gynecologist or your GP for further evaluation of your condition and to perform tests for the above mentioned conditions which would be in the form of blood tests and you may also require an ultrasound of your pelvis to have a look at your uterus and your ovaries to make sure they are okay.
So all in all it seems nothing serious or anything to worry about however with persistent symptoms I would still like to investigate to be sure.
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