After my experience with this and having searched for and struggled to find answers on the net, I thought I would share this just in case anyone else is, like I was, searching for answers at midnight and getting the run around from NHS 111. I was given Northisterone 5mg 3 x per day after laproscopic excision surgery for a 5cm endometrioma, and severe endometriosis. I had refused all other medical treatments, as I know that in general, I don't tolerate them well. I had a few weeks of spotting at first and then no bleeding for two months. The GP had advised me that I could reduce the dose to 2 tablets a day once my bleeding has stopped so after two and a half months of compliance, this is what I did. 1 week later I started to get cramps, and bleed. The cramps increased in severity throughout the day - they were quite sharp and caused me to double over. There was no pain at other times. After a severe cramp I felt like I was passing a large amount of blood, but looking at the pad I was wearing, I saw a two inch long by about an inch wide of a tissue like substance, greyish in colour with tiny blood vessels running through it. It had no particular form. The following day I took this to our local urgent care centre, where after taking a full history and examining the tissue in suspension, the doctor said that it looked very much like uterine tissue, she was quite sure, but has sent the sample to histology for confirmation.
From what I can glean from what she told me and doing more educated research on the net, this is a rare side effect of progesterone treatment, and can result in expulsion of the entire uterine lining in one piece. I only expelled a small portion, but It was a very unsettling experience, compounded by the fact that I couldn't find any definite information on the net. As far as I know, this isn't dangerous, but it is rare - no mention of the possibility of this in the medication leaflet.
If the histology results show anything different to what is expected, or if my consultant has more to add on the subject, I will reply to this post and let you know. I just wanted to leave this out there so that people can refer to it, and not be simply diagnosed with early miscarriage as others apparently have.