How quick can you fall pregnant after coming off the pill?

I stopped my pill about 4 weeks ago, I’m yet to have a period but in the last 5 days my boobs have been really painful, swollen and sensitive. I’ve been more emotional than usual and very tired. I understand this could just be signs of my period being due soon but wondered if anyone else has fallen pregnant quite quickly after stopping the pill and if you HAVE to have had a period after coming off the pill before you can fall? (Hope this makes sense!)