dilute urine

I received a call from HR today stating that my pre employment drug sample was inconclusive-dilute. I was just wondering if anyone else has experienced this and what I can do to ensure that the next sample is not diluted. I drink a lot of water but since finding this out, I have not had hardly anything to drink. I plan to go first thing in the morning to test and use the first urine of the day. I know that my test is negative, I just don't want to chance it being dilute again.
you have pretty much nailed it, overhydration, loweres specific gravity

ok so should it be ok/back to normal tomorrow

Yes, go back and just do not drink before you urinate. We did not even have sinks in the bathrooms where we gave our samples so they could not be watered down. If you think someone is playing with your sample ask for a blood drug screen as well, that should confirm you have nothing in your system.