You are born with freckles and they don't just develop. Get the other ones checked.

An increase in number of freckles usually indicates increased sun exposure or sometimes UVA or UVB administered in a light box at a derm clinic or even in a tanning bed. Because there is increased Ultraviolet exposure there is also an increased risk for skin cancer. You should visit a board certified dermatologist for a skin cancer screening and mole exam to be safe.