Every time I flush my toilet my kitchen sink fills up. Why is this happening?

Is it 2 story house ? Toilet upstairs ?
Back to you. Milo

Yes this is a two story house and yes, the bathroom is upstairs

Interesting. I think you nailed it Milo. Only way this could be happening is if the bathroom is on a level higher than the sink.

Guess we need to let Milo finish the explanation we three are thinking

Make it four.

And when Tom gets here in the morning it will be 5

One more question: Is the house on slab or on raised foundations ?

Back to you. Milo

Raised foundation

If this were my call I'd be headed to the roof vents with a sewer machine.(A outside cleanout would be nice). Sure sounds like a partial blokage downstream from the kitchen sink. Good call Milo. Tom

Tom said it: clean your main drain. There should be a Clean-Out either under the house or somewhere on the wall hidden by a round cover. It is a routine service call by a drain cleaning contractor.

Since it is only kitchen sink that overflows - and not any of your downstairs plumbing fixtures - than that tells me the sink is the first fixture connected to the upstairs toilet drain pipe under the house and blockage is somewhere right behind this connection.

Back to you... Milo