You may need to flush your hot water heater.
What Does It Mean When Brown Water Is Coming Out Of The Faucet?
You need to run the water a little because the water has probably been sitting in the pipes for a while.
tht either your pipes are dirty or your sewer is backed up ... Either way its not good ... But to clean it just boil it for about 20 minutes..
Could be that you have had some sediment build up in the pipes and it has broken loose.
It means that either the pipes are rusty in your sink, or that there is rust in the hot water heater or the lines soming from the hot water heater.