I can't take birth control

I have a rare autoimmune disorder called antiphospholipid antibody syndrome and because of it I had a stroke and so I cannot have added estrogen in my body. My boyfriend never knows when he's about to ejaculate and says he only ever has once but still we use condoms, however we forgot to use one after getting caught up in the moment. I was ovulating the day it happened and now am showing signs of pregnancy, could it be because i was panicked and its now a placebo effect? What would the chances of pregnancy from precum be? Are their any other forms of birth control that don't raise the risk of blood clots that I can go on to prevent a possible accidental pregnancy?