Hello,
I have gone though your query and understood the concern. As you did not have any surgical intervention, the possibility of uterine synechiae is very less. The possible reason for the decline in the menstrual flow could be due to a change in the hormone influence on the endometrium. So long as you are ovulating normally and the tubes remain patent, you need not worry about the duration of the menstrual flow. Altered patterns are common during the reproductive phase. If you performed the HPT at least a week after the missed period, pregnancy is unlikely. You should have a proper clinical evaluation along with a pelvic/trans-vaginal sonogram to plan further management. Hope this helps.