Hey mummy don't be disheartened! First of all, periods can get in the way of your milk supply so you may notice a drop whenever your period is coming :) It's also normal for you to miss periods here and there, or for it to be irregular at times - but always remember you CAN still get pregnant even if your period doesn't come!
As for why your milk supply drops, there are many reasons to it - some common ones are not pumping often enough (should still be once every 3 hours to keep up with demand, if there's no demand for milk then your body would start telling itself to produce less to meet the reduced demand), bad eating habits, not drinking enough water, not resting enough/well, having stress, blocked milk ducts etc.
If you're having any of those stated above, get them resolved so that your body can do what it needs to do :)
Jorelle Aw