Hi Ran, I'm really glad to hear from you again, and that you were able to get your surgery on Saturday. I can't comment on the nausea/diarrhea situation, but I do think it is normal to pass a small amount of tissue/clots after SM. I am one week post-op today and so am in a very similar situation to yourself. I have passed around 10~ small (a couple of mm to 1cm max) clots over the past days, and also one piece of thin 'tissue'.
I also felt a bit frustrated and that the surgery was meant to 'remove everything', but after doing some more research I understand a bit more about what's happened, and why it's for the patient's health, so hopefully this explanation helps you as well.
When a first ERPC is conducted, it is done as a 'blind procedure' (hopefully this was told to you before your op), so basically no u/s/imaging devices are used to guide the surgeon to show exactly what tissue is being removed. As such, the operation is done on a very light touch basis to avoid any damage to the uterus. Therefore, it's very normal that some small bits of tissue could be left behind and will naturally pass in the days following the op via some bleeding/cramping.
How ladies experience this bleeding can be in many different forms (there can be some days delay before it starts, there may not be very much etc) but essentially all bleeding should be following a pattern of getting less over a period of around two weeks. Having some pain during this time is also totally normal as the uterus is contracting back to its pre-pregnancy size. This also helps with emptying out anything that is left behind.
Sorry if TMI but I had horrible constipation after my op, and didn't have a BM until about four days after. I can't comment on your nausea/diarrhea, but as you've already have investigated, these can sometimes be signs of an infection which you've had ruled out so that is reassuring. I've had a look just googling 'diarrhea erpc' and it appears that Misoprostol (the tablets you were probably given to dilate your cervix prior to the op) can sometimes cause that; I'm not sure if you're on any other medications, but you should check the side effects of those too.
In summary, it is normal to pass some small amount of tissue and clots after the surgery, and any immediately sinister reason to your diarrhea has hopefully been ruled out. Most importantly, are you still in any pain? If not, then that's hopefully a good sign, although the back pain could have been linked to the reasons I gave above. I hope you're able to speak to your doctor today and they can give you some further reassurance, but it sounds as if what you're experiencing (provided the pain is no longer horrendous) might be within the realms of normal.
If they are extra concerned, they may offer you another u/s to see if anything large has been left behind which you would not be able to pass yourself, or could be behind your symptoms, but I hope you are given the all clear.
I know my experience is anecdotal, but I also felt very good the day immediately after the op (although I have bled from that time onwards until below) and it was only about 24-48h post that I started to go through some cramping etc. I was very weak the first couple of days and could barely go for a walk around the garden. However, after this, my energy/vitality has come back day by day, and I've noticed, particularly over the past couple of days, that the bleeding has really started to lessen, and nothing came out on my pad last night or this morning when I went to the toilet. As such, I'm very hopeful that things are starting to return to normal.
Please let us know how you get on xx