Hi,
There are numerous reasons why your menstruation cycle can be abnormal such as this, Pregnancy and stress being the most common.
Since you already tested and got a negative result, you also should consider whether or not you've recently been sick, gained or lost a significant amount of weight, have been under excessive stress or have taken any new medications. All of these things can affect your menstrual cycle and cause you to have irregular periods.
Since your now working a night shift your body is going though a change and this is the likely cause.
I would bring it up at your next doctors appointment, if you continue to have abnormal cycles it is a good idea to set up an appointment with your doctor to determine the cause and begin any needed treatments.
Walter
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