Toilet leaks from underneath when sink faucet runs

My toilet is only three years old and there is no crack in the base. When I removed the toilet there was water in the line. I poured the liquid main line cleaner down the line and when water is poured down it it does not back up. On the other hand when the sink is run it backs up to the main line where the toilet attaches. So at this point I have not reattached the toilet to prevent water coming out from underneath. This is alos located on the first floor. Does anyone know what may cause this?


You main line is clogged and you need a plumber to come and clean it.
Lay of the chemicals and tell the plumber what you used.