First of all, congratulations!
There is an itchy rash which pregnant women get near the end of their term. No doubt that is what your doctors thought you had. However, that would not reappear after delivery.
Rather than just use antihistamines (which can, by the way, be cautiously used during nursing)I think you ought to consult your doctor and perhaps a dermatologist to see whether you have eczema or some other ongoing skin condition which would benefit from more specific treatment. There is in any particular rash associated with nursing that I know of.