In my bathroom I had an old light on the wall that had an outlet on it and the switch was also right on it. I bought a new light and had to run a new wire for the switch for the light. Connected new light ran the new wire and hooked up a new switch. The switch is fine when I have the light on, but when I turn the light off it trips the breaker. I have tried 2 different switches and same problem happens. I have checked the connections... black to black and white to white. No wires are touching metal, house was built in 1950 and it has old wiring and I purchased new romex 14-2 wire. Can't figure out what is making the breaker trip. Switched over fuse box to breakers just about 2 years ago and never had a problem. Switch is on 15Amp breaksr. Any clue to the problem?

Most likely problem is the switch circuit.

The cable that supplies the power for the light comes in behind the fixture, correct?

If so, then you probably ran a "Switch loop" over to the switch and back to the light, correct?

A switch loop uses both the black and white conductors to travel to the switch and back. However, the white is used to carry electricity to the switch and the black is used to return the electricity to the fixture.

Basically, this is what I would expect to find behind the light fixture.

White from supply line to white on fixture. Black on the supply line to White on the switch line. Place a small band of black tape on this white wire at both ends to identify it as a hot wire.

The Black from the switch gets connected to the black on the fixture.

Also, would you mind attaching a photo of the connections you have made in case your wiring plan is different.

Turning Off a switch(opening a circuit) wouldn't/shouldn't/couldn't trip a breaker.
I ran into 1 2 weeks ago.
The homeowner replace a switch with a 3 way switch, and hooked the ground to one of the switch terminals. Turning it off(switching hot to ground) tripped the breaker.
Is this a single pole switch, and does switch have a ground attached?
Donf has the switch loop covered.

I am having problems getting a picture of the connections on here. But it is a single switch not a 3-way.
The connections are as you have described. 2 black wires from the back of the fixture, that is connected to the black main wire and I have connected the black of the new romex all together and the new romex black runs to the new switch. Same goes for the white. When I have the light on it works, then I turn it off and the breaker trips.

Ground was attached but I did take it off and still had the problem.

Now I wonder if the switch is upside down, and light is powered with a constant feed, black to black and white to white.
Turning switch off is really turning switch on, shorting out. Look closely at the small writing on the switch for OFF/ON
Like donf was saying, the black and white that have power, of those 2 wires, white goes to lights neutral, then the black power wire connects to the white that you ran to the switch, then the black that comes from the switch, goes to the lights black wire.
The white going to and from the switch needs to be identified as hot, with black tape at both ends.