Milk supply getting low. Why? Will it come back?

So my LO IS 5 months old and I've been noticing she produces a lot more saliva she even had a rash under her chin and throats area. So I'm guessing she's teething. This started about a month ago. Well now I've noticed my milk supply dropping a bit. I pump at work and then normally bf as she wishes when I get home. I would usually get 16-18 oz in my work hours. But now it's down to 11-14 oz. I'm really worried because I've worked so hard to get it that high. My period also returned last month though this month I'm still waiting. Sorry if this is too confusing. Any advice?