Hello
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This is an irregular period that you had, and can happen once in a while to most women.
Do not worry.
Anxiety, stress, lack of sleep, exercise, weight fluctuations, travelling, smoking , hormonal medications - all these can cause an irregular cycle at times.
The pregnancy test is negative, so there is no need to worry.
If the pattern of irregular periods or spotting persists, you need to consult a gynecologist for a thorough evaluation.
This would include - per vaginum and per speculum examination, a pelvic ultrasound scan and hormonal levels - FSH, LH, TSH, Prolactin, OGTT, DHEAS, Free testosterone.
All the best
Please feel free to discuss further.