Having a baby is a very emotional experience. There are three causes of low mood after childbirth:

• Postnatal (puerperal) psychosis. This is an uncommon but severe form of mental illness. It is so severe that it requires hospital admission and in-patient care. 
• Baby blues. It is very common and almost considered normal to have these. Symptoms include being weepy, irritability, anxiety, and feeling low. They do not usually need any medical treatment. 
• Postnatal depression (PND). This occurs in about 1 in 10 mothers. Symptoms, including low mood, last for much longer than with baby blues. Treatment is advised. 

Signs and symptoms:

The symptoms are similar to those that occur with depression at any other time. They usually include one or more of the following. In PND, symptoms are usually there on most days, for most of the time, for two weeks or more.

• Low mood tends to be worse first thing in the morning.
• Not enjoying anything. Lack of interest in yourself and your baby.
• Lack of motivation to do anything.
• Often feeling tearful.
• Feeling irritable a lot of the time.
• Feelings of guilt, rejection, or not being good enough.
• Forgetting or losing things or being unable to make a decision about things.
• Feeling unable to cope with anything.

When to see a doctor?

Sometimes, self-help measures are not enough. If symptoms persist for more than two weeks, you should consider seeking professional support. Or, if the way you feel is interfering with the way you are used to functioning, you will feel better if you let your doctor know. More often than not, a combination of therapy and antidepressant medication is the most effective treatment for postnatal depression and anxiety.

https://patient.info/mental-health/postnatal-depression-leaflet

https://www.mayoclinic.org/diseases-conditions/postpartum-depression/symptoms-causes/syc-20376617