TTC post partum

I'm 4 1/2 months post partum and I'm trying to conceive, is it true your most fertile during this time? I had pp bleeding for almost 10 weeks took birth control for not even a month ( I had bad side effects) and we just decided to try for #2 since we wanted another anyways why not now. #1 wasn't planned at all but we were having unprotected sex for 2-3 months before I got pregnant. I didn't think I could get pregnant before which is why we didn't use protection. I just stated tracking my period and using this app in hopes of having a speedy conception. Does anyone have any advice or similar situation??