Last night, I had a huge rash the size of my calf. It was very itchy and painful. I admit I did scratch it a lot. I put some lotion on it and took some benadryl. This morning, it is still very painful, a little itchy and the area is covered in bruises. Is

Where ? If it is getting painful and just a little itchy and localized to 1/2 vertically of the body, then one must consider shingles. Call your doctor to find out since the severity of this condition may be reduced by taking antiviral medication in the early stage. If it cross the midline, then you may have something else which also calls for medical attention.
Poison Oak/Ivy. My first thought is that you have a contact dermatitis. Have you brushed up against any plants or perhaps a dog that may have done so?