Hi,
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Pregnancy chances could be possible (as suggested by your symptoms of spotting, lighter period, pelvic pain, low back pain, nausea, headaches) even after tubal ligation due to incidences of spontaneous tubal recanalization (especially if you are having functional ovaries and if you are sexually active). Due to tubal scarring, chances of ectopic pregnancy become high. If pregnancy is suspected, visit your doctor/gynecologist at the earliest for further evaluation (especially to rule out ectopic pregnancy). If pregnancy gets ruled out, further evaluation would be done to look for causes like menopausal onset, hormonal derangements, thyroid abnormalities, ovarian cysts, endometrial changes etc. Treatment as appropriate (based on examination findings and investigations results) would be started to help you control the symptoms and regulate your periods. Be in regular monitoring and follow-up with your treating doctor/gynecologist and report any new/abnormal symptoms immediately.
Take care.