I think itchiness can be a symptom of lowgrade irritation, and I imagine this would apply to the vagina as well. You could try using lube and see if that helps.

It is possible to be allergic to semen (though from what I understand, that usually causes a burning sensation). However, if he's coming inside you, semen changes the pH of the vagina in a way that makes it more hospitable for yeast to grow. I don't know if that's a factor, but it could be. At any rate, you could ask your doctor.