Hi mama,
Here’s an article on your period returning and a drop in BM supply:
https://www.llli.org/breastfeeding-info/menstruation/
It’s normal for most mamas to experience this. Is baby producing at least 5-6 wet diapers and gaining weight well? If so, keep up the good work mama!
Baby could also be fussing due to growth spurts during the 4 month and 6 months mark.
To increase milk supply try:
1) having a nursing holiday - nurse baby frequently and relax with lots of fluids (cooling coconut water is good too) and your fave tv shows
2) lots of skin to skin
3) breast massages
Here’s an article which may be helpful to you:
https://kellymom.com/hot-topics/low-supply/
Hang in there mama.