Thanks for the query to ATD for an opinion,
At 21 weeks pregnancy, it is difficult to explain this very symptom of vibrating vagina here. There is a possibility of a preterm labour or a premature rupture of membranes or there may be possible vaginal fungal infection which is causing an itching feel which can be perceived as vibration or for that matter it can just be a transmitted Braxton Hick contraction, which is unlikely though at 21 weeks.
In either case it is suggested that you visit ER to get a complete examination done by the physician both per speculum to rule out any leakage or infection and also an ultrasound pelvis to ascertain foetal well being.
Wishing you safe pregnancy,
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