I had really low fluid a little bit later in pregnancy and basically I didn't drink enough! or anything really
I started drinking 3 1/2 ltrs a day, until my levels got back to normal & then made sure that I drank 2-3 ltrs a day. I went from 8 to 10.5 on the first scan & then to 12 on the third.
Also rest lots, put your feet up if you can, don't get too hot, you can build up your fluid so cross fingers this works for you.