It appears that you have a sebaceous cyst probably from an infected hair follicle. This is nothing serious, just a nuisance. With it itching and burning it may be infected with bacteria as well as a fungus.
Make an appointment to see your doctor to get the right treatment. You will require an antibiotic if the cyst has a bacterial infection. For a fungal infection (this is quite common with rampant sex) you will need an antifungal medication either a cream or a one dose oral capsule called Fluconazole.
As a home remedy to help stop the itching and burning sensation, apply natural set yogurt. This will help. Do not use soaps to wash down below because this changes the pH level of the skin that can also lead to problems.
It does not mean that just because you have a bacterial or fungal problem in the private area that you have been unfaithful. Men can also get fungal problems and again it does not mean that they have been unfaithful.
If your diet consists of a lot of sugar and refined processed foods, or you have been on any antibiotics, this will encourage yeast growth on the body.
Only wear cotton underwear and do not wear tight clothing or nylons tights.
Safest will be to consult doctor and get all the test done for both. This will relieve you from the worries. Both can have food confidence in each other. otherwise you will be suspecting on each other and destroy your marred life.Your narration of symptoms does not indicate that there is any STD.