My baby is 5 months and she has developed rash around her neck and its spreading to her head, forehead, shoulders and some parts on her body?

Too vague for label: Rashes are common in childhood & can be triggered during/after illness, as an allergic response to foods or contact with a variety of agents. The description provided is too vague for a label. You can start over & provide a better description of the rash or just have someone evaluate it. These often require a visual inspection to unravel the potential cause.