Hi I have a IUD (coil) fitted and my periods are supposed to be really heavy because of this which is fine. But this past week my period has been light and brown. What could this be and could I be pregnant? It is supposed to be 99% effective.

No one can possibly know if this is just stress, lacking diet, low iron, or an unusual hormone fluctuation - or pregnancy. You will just have to wait until you can test. Odds are you aren't pregnant, but if you fall in that group, pregnant is pregnant.

There is no form of birth control that's 100%. 99% isn't 100, so there's always a chance of pregnancy any time you have sex.

Could you be pregnant? Yes. Are you? We don't know. You'll have to wait at least 2 weeks, then test using first morning urine.