dropped supply? supplementing?

ok so I'm pumped (haha) and have gotten almost 11months! I just got back from a vacation (5days) and had to pump and of course couldn't keep up with what he drank while I was gone. 

So I'm assuming I'm either pregnant, about to have my period, or...the trip killed my supply.  He's teething so its hard to tell fussy vs hunger.  I just nursed him and he acted finished, but about 10-15 minutes later he was super clingy (like he is when he's hungry)....so I gave him some milk I had recently pumped...he took 5 ounces.

So...I'm wondering if I should start supplementing maybe during the day and keep up the am/pm nursing? 

Thoughts?  I would have no clue how to introduce formula mix with some breastmilk...how much to give him etc...