Is this normal after having Radio active iodine treatment for your thyroid?

Ok, so back at the beginning of february I drank some radio active iodine solution to oblete my thyroid which pretty much means to make it inactive because it was over active plus I have Grave's disease. I have noticed that since the treatment that my monthly cycles have been very irregular. As in I am getting a lot of them and they seem to be coming whenever they want. It is like I will have my period the week that it is due and then 2 to 4 days after I will have another one. Or sometimes I will start having my period the week before it is due and will be on it for almost 8 days. I am also on birth control pills. To those of you who have had experience with this do you think that body is just thrown off by the changes going on with my thyroid or do you think that something else could be going on? My thyroid doctor seems to think that it is the changes in my thyroid but I am beginning to wonder if maybe there could be something else causing it. I mean i feel fine otherwise but it has me wondering. So, I am looking for some input. Thanks.