My one nipple is really itchy and sore what does that mean?

Itchy and painful: nipple. Could be due to skin irritation, infection, if associted with any changes in color, texture or inversion of nipple could be suspicious for CA. Examination will be helpful

Itchy nipple: Could be eczema or yeast infection of nipples. Might consider topical antifungal like clotrimazole which is safe in pregnancy but would discuss any medication use with your OB . Breast tenderness is common is pregnancy, but would discuss your particular case with your OB as you are pregnant and do not want to miss other causes like infection,duct blockage.