Should I quit my job?

Long story short: I have been filling in on a lot of the responsibilities that were assigned to a guy who got fired in December, even though my boss officially offered me his job and I said that I appreciated it but was very happy with what I was doing at the time. They finally hired someone else in early April. At the end of the new person's first week my boss called me into his office and said that the new woman and I would have the same job and that we would work together. The new woman's salary is $7,000 more than mine. I feel extremely taken advantage of. Should I start looking for a new job? Should I ask for more money? Or should I realize that this is just one of those things that is not fair? My title, incidentally, is Administrative Assistant and the new woman's title is Analyst.