Is it possible (regarding going from pumping 5 days a week to exclusively feeding)?

Because she is nursing now - which is stronger on the stimulation of the nipple then the pump it will increase your prolactin levels - the increase of the prolactin levels could be causing a disturbance in your cycle. The other aspect to think about - if you were not using birth control (to regulate your period) then it is highly likely that your menstrual cycle was close with other colleagues (it is a hormone thing), now that those colleagues are no longer around your body will need to make its own schedule.