What might it mean if a 49 year old woman gets her second dose of pfizer on her period and ends up having a longer than average period, with steady flow for 9-10 days?

Menstrual hormones can fluctuate if a woman's body is affected by stress, lack of sleep, weight gain/loss, increased exercise or illness. The Covid vaccine can be considered a stressor, since it causes a large immune response accompanied frequently by enlarged lymph nodes, fever and other symptoms. Most probably your hormones fluctuated in response to this stress causing a longer period.