I have rash spreading through my body?

I am a healthy 33 year old with no known allergies or diseases and I workout daily. About 10-12 days ago, I felt itchy between my breasts and thought nothing of it. About 2 days later, I had a full blown rash all over my belly, back, shoulders, chest, and breasts. I went to immediate care the next day and all she did was take a look and prescribed me amoxicillian. 3 Days after that, the rash was not getting better and appeared to be spreading more so I got an appointment with a general doctor and he had no clue what it could be so he prescribed me Hydroxyzine, Methylpredinsolone, and Triamcinolone. Now here I am, 4 days later and still no improvement and appears to be spreading even more into my groin area. It is more itchy in the evenings. I am worried that it may be a systemic problem and I don’t like that these doctors are just trying to treat the systems instead of doing tests to find the underlying problem. Would you be able to help? The rash appears to be irregular shaped, varying in sizes, biggest spot is about the size of a quarter. It is light pink to light red in color. I have not been in contact with anything unusual, no new foods, no new soap or shampoo or lotion.