Honestly, it's most likely leftover urine. If you're well hydrated, it may not necessarily smell like much more than water, and it's harder to empty your bladder entirely when pregnant.
This happened with my first pregnancy. I swore it couldn't be pee, but I went in and got tested and it wasn't amniotic fluid. It also happened occasionally as I got further along. The baby just puts a lot of pressure on all your organs down there and a little urinary leakage is pretty much inevitable for most women at some point.