6 months no period !
I didn't have one for three months after surgery and then it was irregular for the next six months or so. I think it's common for it to be irregular after surgery although many people seem to have it more often than usual, not less often.
You might want to mention it to your doc, but it's probably nothing to worry about. If your surgery was six months ago, though, it's time for your six month labs so you can ask your doctor about it when you get those done.
Please note: I AM NOT A DOCTOR. If you want medical advice, talk to your doctor. Whatever I post, there is probably some surgeon or other health care provider somewhere that disagrees with me. If you want to know what your surgeon thinks, then ask him or her. Check out my blog.