car problem

I'm new, so I dont know if this is the best place to post for help, but here it goes...

When I'm getting low on fuel (say around 10 litres (3 ga) or less) and I'm driving up a hill, my car hesitates. It feels like its running out of gas. The fuel gauge also goes down when I'm driving uphill.

Any ideas of what it could be?

I have a 2001 Kia Rio automatic

Thanks...