First of all we should describe what are heavy periods?
The correct medical definition of heavy periods is the passage of more than 80ml of blood each period.
Periods are considered heavy when:
1) a woman bleeds for more than 8 to 10 days, especially if this is repeated month after month.
2) a woman bleeds so much that it is difficult for her to attend her job. She may be forced to plan her holidays and leisure time according to the timings of her period.
3) the bleeding is continuously so heavy that the woman becomes anemic.
4) the presence of other than small clots for more than one or two days suggests heavy periods.
5) 'flooding' describes the sudden, unexpected onset of periods, like turning on a tap, and indicates heavy periods.
Secondly we see side effect in Iron overdose. However heavy period is not reported as a side effect of Iron supplements.
For more information you can see
http://www.danidaily.com/health-and-fitness/iron-supplements.php
http://www.danidaily.com/health-and-fitness/heavy-bleeding.php