postpartum mom without a period

so I am al most 7 months postpartum. I breast feed still, but I know I am getting low and will dry up soon due to work and school and not being able to put p or feed enough to keep my supply. I haven't gotten my period back yet. well on Saturday (the 11th), my husband did not pull out fast enough and finished completely inside of me. but since I have yet to get my period, I don't know when I should test. I don't know when I would miss a period, because I haven't gotten one yet. with my first child, I got my period back at 8 months but I had a way better supply and breastfed more often. so, when do I test, and what are the chances I could be pregnant?