Breastfeeding going onto cows milk 13 months

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Shes now 13 months and the last 3 days shes not wanted the boob at all and got upset if I try she just wants a cuddle at night instead.

I'm now worried she's not getting milk but also have a huge guilt she won't be getting her antibodies from it

I havent expressed and if I'm totally honest the thought worries me as I don't this before but my son got a bacterial infection so it's worried about the storing the milk and heating it ect so I've never done it.

Just wondered what others have done