Can milk secretion stop suddenly?

Q: We have a newborn (3.5 months old now) and my wife breastfeeds the baby. Of late, last couple of days, she has suddenly stopped lactating and is not getting enough milk. We have started our son on formula during the day and my wife is only able to feed him at night. I have a few queries: 1) Does this happen normally? 2) Both of us work full time, could this be a reason. She used to extract milk till last week and of late she is not able to do so as there is not enough milk collected for extraction. 3) At what stage is it safe to start giving the baby semi-solid foods like rice, cereal/mashed daal and fruit. Please let us know at your earliest convenience.

A:1) Yes, this can happen.2) Milk secretion may decrease related to her working full time.3) 3-4 months at the earliest, preferable at 4 months.