Hi, Thanks for your query, after going through history, it may be a viral infection in which there is severe myalgia associated (means severe muscle weakness) , for which just a supportive treatment is required and it may take time of a week to resolve . Still few things can be more focused after going through blood investigations in detail.
Next Steps
Focus on feeding baby with high calorie diet so easy for baby to get free energy source.and can continue vitamine
b12 supplement
Health Tips
If it still persist or if baby starts refusing feeds ,kindly consult immediately.