
Is this her first season?
When she's been laying down for a while, dab her vulva with a white tissue to check for spotting of watery blood - initial bleeding is not necessarily very obvious. The vulva itself will swell considerably during the season - if this isn't yet evident it may be that things will change soon over the next few days.
Keep an eye on the discharge for change of colour - what you describe sounds nothing to worry about but what is normal for some dogs/bitches can be a cause for concern in others so remain aware of her behaviour, drinking habits, anything really, and should the discharge become thick, profuse or smelly then have her checked by your vet to rule out potential infection.
One of my girls had her first season at 21 months and was a bit moody and clingy for around a week beforehand plus also had a slight opaque discharge too prior to the first signs of bleeding.
HTH, Teri :)