I developed a rash on my legs after purchasing a new pair of boots. Could it be eczema? Or jist an allergic reaction?

Not an allergy. Unless you are allergic to latex, it is unlikely you are allergic. Most likely it is something known as irritant contact dermatitis. You have to ask yourself if you have ever worn the same boots, same brand, same style of boots? If so, and no reaction in the past, then not allergic. To be totally sure see an allergist who could perform patch testing.