Brief Answer:
NEED MORE DETAILS

Detailed Answer:
Hi
Greetings
You have given only your age and not pregnant.
Very difficult to comment without knowing details like ,
1.how was your previous six cycles regular or not ?
2.weight and height or BMI
3.Any other hormonal or medical problems
4. Taking any medicines
5 Are you sexually active?
6.If yes how did you test your pregnancy,urine or blood test?
7.Abdominsl ultrasound scan done before?

Please give details of above questions.
In general a girl of 21 years ,not sexually active the irregular cycles could be due to hormonal causes like Thyroid , prolactin or due to policystic ovarian desease.

Feel free to ask your doubts with more details.

Regards

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