In many, if not most, startups and small companies titles don't mean much. You have to do more than you were usually hired for.

Try and learn as much as you can as fast as you can and then either ask for more money or look for another job.

As long as the environment is not bad, it may be a good learning experience. When I compare my startup years to some of my co-workers who never worked at a startup I was able to learn a lot more than them. Many of my co-workers have only worked at large companies so they were never exposed to a wide range of tasks because in larger companies jobs are far more segregated and specialized.