First of all don't panic!

Low amniotic fluid is diagnosed on the index as equal to or less than 5, so you're only JUST into that level. Make sure you're well-hydrated and make an appointment to see the Ob.

Low amniotic fluid CAN indicate a problem with the baby, with it's kidneys or urinary tract. But it often doesn't mean anything, and is a very controversial diagnostic tool due to the number of people who are sent with urgent needs for scans/tests for low AF and are actually carrying what turns out to be a perfectly normal healthy pregnancy that just happened to have a bit less AF than usual that day. Also bear in mind that accurate measurement of AF depends on an experienced sonographer. It is a skill, akin to measuring how much water there is in a glass bath viewed from below with a person in it! Not easy.

So stay hydrated, see the ob, and see what they say. It is likely there's nothing wrong, but getting it checked out is best for you both.

Bx