FBM inventory keeps changing overnight

Can someone help please, it is driving me crazy. All of a sudden my FBM available inventory figures keep changing. I can put 5 units in stock tonight, save it, and then wake up tomorrow and it is back to zero. Or flip side I reduce my stock to zero and next day the stock is back up to 10. Does anyone know why this would happen? I tried to search for advice but all I could find was people suggesting this was rogue traders ordering stock and then cancelling to tie up stock. I don't think this is the case here as there are no cancelled orders. It is causing me all sorts of problems, I had to cancel a number of customer orders this week because of it. PLEASE HELP.