my car sways sometimes on one side what can cause this?

You need to check the suspension with steering

Tie rod ends, sway bar links, worn bushing (control arms, sway bar), there's a few causes, let an alignment shop look at it. Any shop can look at it, but some repairs will require an alignment afterwards, and if that shop can't do alignments, then you'll have to go to the added trouble of taking it to another shop that does do alignments. So save that extra step and just take it to an alignment shop.

More than likely you may need a wheel alignment!

First thing I always do in cases like this is to check the tires for correct pressure as recommended in the car's manual/tire wall, for wear. Then check the sway bars (listen for any noise coming from under the front of the vehicle when it sways), and last do a wheel alignment. I always try to rotate the tires on my vehicles every 5,000 miles.