Hi welcome to healthcare magic forum. You haven't missed any period and your pregnancy
test is negative, hence chances of being pregnant seem remote. Soreness of nipples and feeling tired maybe due to unrelated causes.
I'd like to give a suggestion: Considering you already have 2 children, why suffer repeated abortions? A wiser option would be to use contraceptives to prevent unwanted pregnancy, oral pills
will suffice for this, it has the added benefit of regular periods with scanty flow, in other words, less blood loss every month.