I have red patches on my belly, chest, and lower back. It's been about a week but I don't know what it is?

It is difficult to determine the exact cause of the red patches on your stomach, chest, and lower back without seeing them in person. It could potentially be eczema, an allergic reaction, or even a fungal infection. I would recommend using a fragrance-free moisturizer to help alleviate any dryness or redness. If the symptoms worsen or new patches appear, it would be best to consult with a pharmacist or doctor. If you experience fever, swelling, heat, or fatigue, it is important to seek medical advice promptly.