Vaginal irritation can be due to a number of causes. A very common cause is a fungal infection, where the usual complaint is one of itching, burning and a white "cheesy" discharge. Various types of bacteria can also cause an infection, in which case the discharge tends to be foul smelling and not white. Irritation from items used in the genital area such as bubble baths or douches can cause this as well.
As the causes are many, and the treatment is different for each cause, it would be good to see your physician for diagnosis and treatment. If you are unable to get an appointment before your immersion, and you are concerned about having relations, it is legitimate to delay having relations on mikveh night. Other forms of physical affection are of course permitted; you and your husband should discuss your expectations ahead of time so that mikveh night is a positive experience for both of you even without relations.
Refuah shleimah!