If it smells like ammonia, it is probably urine (embarrassing, but common if the baby is sitting on your bladdar). If it has a sweeter smell, it is probably amniotic fluid.
I'd call the doc, unless it really smells like ammonia. From personal experience, I had a slow leak (a little further along than you) and when it finally got too low, I went into premature labor....we ended up with an emergency C-Section b/c with out the cushion of the amniotic fluid, the cord kept crimping and cutting off oxygen to the baby. Had I gone earlier, it may have at least kept me from having the C-Section.
Best to you!! I know with my 2nd I had incontinence problems and I was scared to death it was a leak...it never really smelled like urine, but it had an ammonia smell which made me feel better! Never thought I would be happy to pee on myself! :-)