There absolutely is a chance you could be pregnant if you have had unprotected sex in the last month - since you stopped using birth control.

how early do mood swings start

There is a possibility that you could be pregnant if you had sex (without any other form of protection) after you quit using the patch.

But, your mood swings could be a result of your body adjusting to the lack of hormones from the patch. I quit using hormonal birth control for 3 months and I had mood swings and was not pregnant.

Also it would probably be a little early to be experiencing mood swings from being pregnant if you are.