earliest I can test?

I have a 7 month old and I haven't gotten my period back yet (still breastfeeding as well) so hard to keep track of anything right now. 
My husband and I had a oops moment about 3 weeks ago so I am wondering when would be the earliest I can test to even get a faint line. 
I only wanted 2 kids and i was in the process of finding a good birth control that worked for me so I am hoping to see a negative! 
(I know there's a lot of people who want a baby and such so please don't bash me for hoping I am not pregnant. I know my limit of what I can handle and that is only 2 kids at the moment) 
Thank you!