The other posters have given some great advice and information. I just wanted to say that having heavy periods, you ought to get an internal and external ultrasound as well as having a full hormone test done. Heavy cycles can result from an imbalance of hormones.
You should also get your thyroid checked. Thyroid related issues are very common and unfortunately so many people go undiagnosed for far too long. Like some of the other posters, i also have Hashimoto's, but it took me 9 long years to get diagnosed. Prior to this diagnosis i kept being told my thyroid was "fine". During this time my ferritin was on a slow decline, even before my periods got heavy, but as i'd never been tested for ferritin before, i had no idea what ferritin was or where i was supposed to be. Even later when labs showed it declining i didn't think it was important as my doctor never said anything. I now realise i ought to be begun educating myself a LONG time ago.
No matter what testing you get done, if your doctor is simply going to look at "ranges" rather than where you ought to be at within your lab results, of course they will keep saying "fine" and you wont ever know there is a potential problem in the making until it's too late.
Your ferritin issue could be as simple as too much blood loss due to your periods, but i'd still get all other possibilities checked out as it's quite common for monthly cycle issues, low ferritin, low Vit D, low B12, thyroid issues, malabsorption of nutrients to all be interlinked. Also like already mentioned, another common one is celiacs.
With celiacs, not everyone will have symptoms so you would never know you had a problem with gluten. Celiac blood tests can be false negative/positive. Only way to know for sure is a biopsy of the small intestine. IBS is a symptom of Celiacs but also can be due to digestive issues eg: inability to digest fats or carbs as well as lactose intolerance. IBS is also linked to a build up of candida in the gut.
In regards to auto immune disease, whether you have a gluten intolerance or not, gluten is an enemy rather than a friend. Best to keep it to a minimum or cut it out altogether. Someone i know has been anemic off and on for most of her adult life, but after cutting out gluten for 6 months her ferritin is now 110. Her gluten antibodies were off the scale so she didn't bother with the biopsy. I guess her new test results for ferritin and her overall health speak for itself.
It might be a good idea to check your Vitamin d levels as well as B12. If it's a malabsorption issues eg: due to IBS, then you could be low in any number of nutrients. You can do a simple experiment. Cut out all dairy from animals for a couple of weeks and see how the IBS goes. If it still continues then perhaps it's not that, but then again lactose is linked to inability to digest carbohydrates. You can look it up on the net. Different processes are required for digestion of carbs, fats and protein. In my case i don't digest protein foods properly due to lack of digestive enzymes being produced by my pancreas so i take digestive enzymes with all my meals.
If the IBS is not due to celiacs or lactose intolerance you can try taking a course of probiotics to help balance your good intestinal flora. Imbalances occur due to antibiotics, birth control, too much sugar or refined foods in your diet, illness, yeast, alcohol, unhealthy lifestyle etc etc as the bad bacteria love the bad foods.
Dont forget that certain foods, beverages and vitamins taken directly before/after/during taking your iron supplement or eating iron rich foods can inhibit iron absorbtion. eg: caffeine type drinks, red wine, zinc, calcium.
Hoping you find some answers soon