Hello from JustAnswer.
A rash that starts in the groin is frequently a fungal infection, but it would be unusual for a fungal infection to become as diffuse as you describe.
It would be more common for a rash this diffuse to be either an allergic rash or a form of eczema
, which can be related to various underlying factors, such as dry skin.
It is possible that this could be a combination of problems, but it is more common that it would be a single condition, so most likely either an allergy or a form of eczema.
There are a couple of options that can be tried at home. It would be OK to take an oral antihistamine, several of which are available over the counter. It also would be OK to try an over the counter hydrocortisone cream. It also frequently will help eczema to hydrate the skin, both by limiting contact with plain water in the shower or bath and the application of a moisturizer. Adding salt to bath water can be hydrating, either as table salt or Epsom salt, but plain water will dehydrate the skin.
If I can provide any further information, please let me know.