Assuming your baby has been gaining appropriately, there is no reason to worry unduly about this nursing behavior. A 3 month old can possibly get 'enough' in just a few minutes. It could mean nothing at all. It also could mean, baby is getting used to bottles and frustrated at the breast. If you have used up 16 ounces you must be giving him lots of bottles...this was in one day? I am confused.
I would suggest, hold off on the bottle. Switch sides or comfort baby another way and offer the breast again later if you think baby is not getting enough.
Dramatic sudden drop in pump output points to pump performance. Trouble shoot your pump. Also, 3-5 ounces is quite a lot to get when pumping and 1.5 is entirely normal. A normal feeed for a 3 month old is anywhere from an onuce to 5 ounces, and many mothers have to pump more than once to get a feed. It is possible you had overproduction and your production has "leveled off" now as is normal, This does not mena you cannot increase pump output when back at work, there are many methods to do that. Kellymom has good articles and I am sure pumping moms here have info.
Also read the article on low production at kellymom, esp the part about what is and IS NOT a sign of low production. If you keep supplementing baby with bottles when it is not needed, that will cause problems.