According to your description of a large reddish patch that is slightly crusty and tender and sore on touch, it sounds as if you are describing an infection rather than cancer. The typical manifestation of the most frequently occuring skin cancer (carcinoma cutis) is ulceration that doesn’t heal.

Another type of skin cancer is melanoma. Melanoma usually manifests with a nevus-like lesion. But that nevus is usually inflamed, painful, itching, bleeding, more pigmented than usual nevi.

Regardless, the only secure method for diagnosing skin cancer is biopsy. You can contact a dermatologist or plastic surgeon about this.