I am not sure what to do about my plumbing problem. I am in a 4 year old house that I bought new. House is two story, and on a hill,so we have a water pump. We have never had any plumbing problems...until now.
Last week the upstairs sink in the bath started getting hot water out of the cold tap and cold out of the hot, but the rest the house was fine. Now, throughout the house, hot comes out of the cold spigot, and cold comes out the hot. Nothing has been touched or tampered with. How does this happen, and what do I do?

Did you put a shut off y on your utility sink or another faucet? If you did and you are shutting the water off from it instead of the faucet you will get a crossover.

As friglet said you have a crossover problem. There is either something that has both hot and cold turned on, but has a secondary shut off that is allowing the cross over or, there is a bad valve. Cross overs could include things such as a utility sink that is turned on both hot and cold but has a hose connected with a turned off nozzle or, a tub/shower valve that has hot and cold on but has a shut off installed at the showerhead and that is shut off. These are common examples of a hot/cold cross connection. Another possibility is that a faucet valve has failed and is allowing the cross connection this is often seen when a moen valve fails, it happens with others too but is most common with moen.

This is a problem that should be taken care of asap. One of the most likely things to have happen is when that hot water hits a cold toilet it may crack.

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Im a headcustodian in suffolk county. I had that happen in one of the schools i worked in. Someone put a shutoff on the utility sink abutting a gang bathroom. Needless to say first urinal on the line cracked big time.

Could be a problem with a recir. system...bad check valve or crossover valve.

It is not a common problem so the fix is not something that can be diagnosed remotely. A plumber, or someone, has to be at the house and trace the water movement through the pipes.