Hmm. Women with low supply often take a drug called domperidone which is available by prescription in Canada and by mail order in the U.S. It's for gastric motility but has the side effect of increasing prolactin and hence milk supply. I mention it because for many women the prolactin boost keeps your period away. I didn't get a PP period with DS1 until I quit the dom when he was about 18 months. The normal starting dose is 90 mg a day but I'd weaned down to 30 mg a day and still no period. It might be worth a shot in your case.
I've also heard that magnesium supplements can prevent period-related supply dips.
Finally, if your period returns because your babe starts STTN or eating a lot of solids, co-sleeping (if you're not already) and going slow with solids might help. Unfortunately, some women just find their period returns earlier than others.
Megan, loving her sweet boys, born Aug. 2008 and Feb. 2011, and their sister, born still March 2007