There should be no problem with you wearing a tampon during the surgery,make sure you tell the anaesthetist that you are menstruating and have a tampon inserted when he comes to see you before the op.
When I had my surgery, they said a tampon was no problem, but they did ask me to wear a sanitary towel too. In the event of an anaesthetic emergency it alerts the team to the fact that you are menstruating and to check for a tampon (which, as we know, can cause a serious infection if left in for too long).
They usually make you take your underwear off during surgery so wearing a sanitary towel could be problematic. I forgot to take my knickers off when I had my lung operation......who would have thought they'd even notice???lol
When I came round after the surgery I was completely baffled as to why I had no knickers on and got quite angry about it(anaesthetic effects hadn't worn off),I was told they had cut them off me and thrown them away,I wouldn't settle until one of the nurses found me some to put on.pmsl.