Ummm... going to need a little more info. Have you done anything different in the last week with your tank? Added any new fish? Added any medication? Last time since you changed the water? You get the idea...
My standby advice in a situation like this: a 20% water change never hurts.
[Edit: Is this the same tank we were talking about here: Bad levels
If so, then it could just be a bacterial bloom from the tank cycling. How are your ammonia/nitrite levels?]