Periods!

Ok so this is my second baby. I’m 4 months pp. with my first I had gotten my period back 6 weeks pp (exclusively pumping) and now this time I got my period 6 weeks pp (October) and then I bled for a few days in November and then December no bleeding/period and as of now, January, no bleeding/period and I’ve taken COUNTLESS pregnancy tests and ALL negative. My thyroid is normal but my parathyroids are low so I was wondering if that could do with any of that? My doctors don’t really have an answer and I have yet to see my endocrinologist. and just wondering if anyone has gone through something similar.