What Causes Missed Period After Taking Pills?

i have been on birth control pills for almost 2 months,when i started the birth control i didn't have my period yet since my sons birth in march, a week after i started the birth control i got my period then 2 weeks later i got my period again, and now 3 days later i have bleeding again and i have lost about 10 pounds in the past month could this be caused my stress or is it possible i am pregnant again?