The drugs she has confessed to using are to induce labour, not to kill the baby.
She believed that her lover may well be the baby's father. Her husband was away when she delivered the baby.
Seems to me that she decided to induce the birth at that particular point in the pregnancy, to ensure that the baby was delivered whilst her husband was away.
She says that the baby was stillborn. That may well be true, but it does seem very convienient for her that this was the case. After all, she wanted to conceal the fact that the baby ever existed from those around her, and the only way she was able to do that was by giving birth alone and selling the baby/abandoning the baby/or burying the baby if it was dead (however the death occured).
I hope that she is telling the truth about the stillbirth, or that she has handed the baby over to loving parents.