a lot of babies go through this phase where they suddenly start drinking less milk than they otherwise did. both my children also went through it.
also, since you mentioned your baby already has started teething, it is possible that she is teething, and hence drinking less due to discomfort. you can keep massaging her gums through the day to ease any pain, swelling or discomfort. also, don't force her to feed. let her take her time and drink when she is comfortable.
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