I have a rash that started small on my stomach and now has spread to my chest and back and now neck. Some people say it's hives. What can it be?

Rashes. Without seeing the rash we would all be guessing. There are hundreds of causes of rashes. If it persists, see your doctor.
Depends. Hives are raised and usually itchy. They usually subside in less than 24 hrs. Are these what you have? If not, then they are rash which can be caused by many things including viral infection. If they don't itch, Benadryl (diphenhydramine) will not do you much good. See doctor if the problem persists for more than a few weeks or is getting worse.