(Disclaimer: I am not a doctor, your doctor or your girlfriend's doctor)
It is possible that your girlfriend is experiencing postpartum obsessive-compulsive disorder (OCD). The hormonal changes during and after pregnancy are associated with multiple mental health issues, most prominent postpartum depression, but also the lesser known postpartum anxiety and postpartum OCD.
While the most common symptom of postpartum OCD are obsessive thoughts about harming the infant, another symptom is fear of exposing the child to something harmful, for example diseases or toxins.
If you talk to your girlfriend and get the impression that she is having intrusive thoughts, or can't stop doing what she is doing, or get the impression that she herself is suffering from her actions, try to suggest talking to her doctor (whichever doctor she trusts most - for example her gynecologist, pediatrician or GP can all refer her to a specialist) about it. While the OCD often goes away on its own, the symptoms can be treated.
Again, not saying that this is definitely the case here, but it is something to be aware of. Talking to her about why she does what she is doing (is she afraid the child will die, for example?) showing her that this is not recommended should be the first thing to do.
Estimates of how many women show postpartum OCD vary greatly - I could find estimates between 1 and 10 percent. Unfortunately, most scientific studies I could find about this are behind a pay wall.
Prenatal and Postnatal OCD
Onset and Exacerbation of Obsessive-Compulsive Disorder in Pregnancy and the Postpartum Period