There could be a lot of reasons for your lessened milk supply. If you child was tongue tied (I had one but not as severe), then perhaps your baby's nursing wasn't enough to get your milk to come in more. Also, if you were supplementing with formula, your baby may not be nursing as much from you (regardless of the time given to nurse) as the bottle may be the easier.
Typically milk production is based on demand. The more demand, the more milk.
You may just not be a milk producer. My sister-in-law couldn't nurse her children beyond the first month because her milk supply would dwindle to nothing. My pediatrician brother (her husband) wouldn't believe it was the way her body was programmed (it seemed a inconceivable medical issue). She usually lost milk production in one breast totally by the second week. By the time their third child came along, my brother was convinced of the reality of their situation, and my sister-in-law was ready to switch to the bottle by 6 weeks. Her attitude: at least she got to nurse for a short while giving the baby the best dietary start.
While it does seem coincidental with your period, it may be worth speaking to your OB about it. Try to pump every 2 hours (unless it's feeding time)for 5 minutes on each breast to see if you can get more milk to come in. I would give it a week or two, if you can, to see if your body produces more milk. Also, make sure you're eating and drinking (especially the latter) to increase your odds of producing milk.
I hope this helps.