JennJenn68, I have the same problem!
The hints they gave me were to make sure to take the iron with Vitamin C - it absorbs better. Don't drink caffeine with it - and limit your caffeine altogether. Apparently, it depletes iron.
I would think the time of the month matters as well. When you're lower on blood, your iron is going to be depleted.
If you hear of any really great tips on your end, let me know. I know that frustration of wanting to give so badly and being juuuuuuust under the limit to be able to do so.