I came home and there was a used lady product in the toilet. The toilet is turned off cause it runs constantly so it isn't being used. Is there a plumbing explanation for this or was someone in my house?

You have two problems.
1. You may had a guest and used your toilet
2. you have a leaking problem with the toilet running constanatly, please give us some more details that how and when it is running, it can be a flapper valve problem or the fill valve both are easy to fix, Thanks.

John

I have a korky anti-syphon that isn't working properly so I turned the water off. Can something flushed down the one toilet come up the other?

I came home and there was a used lady product in the toilet
For openers change your locks! Tampons just don't float around looking for a home port. It was placed there. If you had a back up the water in the bowl would have matter in it and would have overflowed on the floor. You didn't say **but** is this a condo? A house? A apartment or flat? Any "roomies" or overnight female guests? Got a girlfriend? Any one with a key to your place? On to your running toilet! Once more you've failed to give any information that could help so I'll have to keep on begging for details. There are two types of running toilets.
1) The ballcock isn't adjusted and allows water to enter the tank and fill up only to run out the white over flow tube.
2) The flapper's shot or the linkage isn't adjusted and the water leaks past the flush valve, (see tank detail) into the toilet bowl.
Which one of the two is it? Regards, Tom

this is what we have. And water is running out the overflow tube

<a href="http://s31.photobucket.com/albums/c373/pizzyp/?action=view&current=P2-04-215.jpg" target="_blank"><img src="http://i31.photobucket.com/albums/c373/pizzyp/P2-04-215.jpg" border="0" alt="Photobucket"></a>



to answer your other questions.

it's a rented house. My girlfriend lives with me and swears its not hers. (so you can see the trouble I'm in). The landlords say they were not here. Asking a plumber is my last hope.

sorry maybe this will work

http://i31.photobucket.com/albums/c373/pizzyp/P2-04-215.jpg

I'm sorry! We posted at trhe same time. I now have theinformation I need to make a answer.
Your Korky Ballcock needs to be adjusted for the water level. Here's how.
1. To adjust, simply turn upper half of valve counter-clockwise to unlock, pull up till water line on valve is level with water line marked in tank, then turn clockwise till valve snaps into locked position.
2. Turn on water supply to fill tank.
Easy huh?

can something flushed down the one toilet come up the other?
Now you got me trolling for information again.
OK! Are the toilets back to back or are they offset from each other? Regards, Tom And again we posted on top of one another.

The toilets are in a house. They are not back to back but are about 6 ft. from each other with two showers in between. By the way, I know how to adjust the korky but to no avail. Thanks for your help!

Then replace the ballcock if you can't adjust the water level.

the toilets are in a house. They are not back to back but are about 6 ft. from each other with two showers in between Perhaps in a back to back situation with a sanitary cross instead of a double combination, and if you had a partial blockage downstream from both toilets, it might be possible for solids to be flushed over to the other toilet but there's no way It's going to happen the way you describe your layout. If it were me I'd hide a few valuables and tell the girlfriend I was hit by a female burglar with a problem. Good luck,(And I mean) GOOD LUCK!! Tom