I had a look online.
The litmus test strips test the PH levels, normal vag PH levels are like 4.5- 5.5 or something and amniotic fluid is 6.5-7.5 so it's obviously higher. So if the strip reads a high amount of PH it'll change colour. However there are a few things that can change the PH and give you a false positive, if your urine is particularly alkaline, vaginal infections, blood or semen.
I don't think you're suppose to test the wet spot in your underwear because of that was a spot of strong wee then it'll show a positive. The study i read said the doctor uses a speculum to take a sample from inside your vag (untainted by urine). I know with my daughter i stuck a long cotton swab up there and they tested off that.
However, regardless of the fact that false positives do exist it would probably be a bit irresponsible to not call your doctor or the labour ward and get checked out.
If you want to test if it happens again. Lay down for a n hour or so and then stand up, amniotic fluid usually pools and then comes out when you stand up. A lot of women said if they flexed the muscles they could usually squish a bit more out, I've peed myseld when I've sneezed or vomitted and it usually can't be replicated with anything but another sneeze.
