Hi,
The fact that all your pregnancy tests have come out to be negative indicates that you are not pregnant. There are some other possibilities like polycystic ovary disease, fibroid (which you arleady have). Some times when a pre-existing fibroid increases in size or new ones come up then a patient might have irregular menes. The other possibility at your age is polycystic ovary disease. To diagnose either of these conditions you need a few blood tests and an ultrasound. I would advise you to see your Ob/Gynae as soon as possible for a detailed investigation.
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Regards
Dr. Gupta