Quoting MAMA JDM YO:" I got pregnant without a period lol my last period was dec 26 & didn't have a period .... Then in may found out i was 7 weeks. didn't bleed at all or spot. So yes its possible."
I think the problem is that once you ovulate, you get a period. Sometimes you get lucky and sperm meets egg before you get your period, but there's no way to predict when ovulation is happening. I've heard of people doing temping while bf'ing to see if they think they might be ovulating, but i don't know how effective that is. Best advice for now if you don't feel like weaning is have a bunch of sex and hope you get lucky.