Allergic to breastfeeding?

Looking for advice. I have been experiencing hives since I gave birth 21/08/24 , been to my GP several times and tried 4 diff types of antihistamines, tried cutting out dairy and gluten ,changed washing powder and avoiding new products on skin. I suggested to my GP it could be the allergy to breast pumping (you can be allergic to the hormone prolactin it produces) but she dismissed me and made me feel stupid. I pumped for an hour yesterday and hives were at their worst. Not sure what to do, don't want to stop expressing milk if that's not causing the hives. Going back to GP today but if anyone has any advice would be great. The hives are really itchy and making my skin break now! On my legs, stomach, back and arms :(