This is an extract from the Summary of Product Characteristics (SmPC) for Tamiflu used by Healthcare Professionals when prescribing:
emc.medicines.org.uk/medicine/10446/SPC/Tamiflu+75mg+hard+capsule/#PREGNANCY
and this is from Relenza:
emc.medicines.org.uk/medicine/2608/SPC/Relenza+5mg+dose+inhalation+powder./#PREGNANCY
So, it seems that there is some animal data regarding pregnancy but little or none regarding lactation for either product.
This is the WHO advice www.dh.gov.uk/en/Healthcare/Children/Maternity/Maternalandinfantnutrition/DH_099965
I agree with Lupusina that calling NHS is a good idea - WHO states that pregnant women should take Relenza due to lower exposure. The Tamiflu SmPC has more relaxed wording around breastfeeding.
If it were me I would be concerned too but I think I would probably take either as that could help me recover more quickly and continuing to feed the baby which could contribute to keeping the baby healthy. They have presumably been prescribed to you because the doctor feels that the benefit outweighs the risk of any potential side effects on your baby.
I hope you feel better soon.