Irregular periods after birth.

I had my little boy in February, I got my first period within 6 weeks after giving birth. I missed a period in April, AF arrived in May, was 19 days late in June, July was normal and now I am three weeks late, I took a few pregnancy tests leading up to AF and on the day of expected period but they was negative, well the last one I took I thought I could see a faint line, I haven't tested since. Only had sex twice with the pull out method. I will take a test soon but I am too scared. We have a 3 year old daughter and a 6 month old son. I have been really exhausted lately. Anybody else having the same problems with their periods after birth? P. S I am not breastfeeding.