Getting pregnant after giving birth

I gave birth June 21st and I haven't started my period yet, I'll be 7 weeks pp next Tuesday. I've been having light cramping like period cramps, but I've been naesous and puking for 2 weeks now. I've been on my birth control alittle over a week now. How possible is it that I could be pregnant if we messed around before I got on birth control? No negative comments please, we would love and accept the baby just the same as the first, it would just be very unexpected!