Spot on advice there from goldmom. I haven't had any issues with pump parts yet (I have the medela swing) but that is definitely the first thing to check out. I do notice quite a dip in my pumping output when AF comes around too. Also consider the times that you are pumping. I get the most first thing in the morning & in the night before going to bed. Since you are also nursing your LO during the day, you won't get too much after the feed.
You could read this link for more information. It also has pointers for increasing your supply.
http://kellymom.com/bf/got-milk/supply-worries/pumping_decrease/Just pasting a few points that could be the reason
Have you reduced the number of pumping or nursing sessions recently, or cut back on nursing/pumping in other ways? Milk production is a demand-supply process. More nursing/pumping results in a greater milk supply. If you consistently decrease nursing or pumping for several days, your overall milk supply will decrease and you can expect to see a decrease in pumped amounts.
Has baby started solids recently? As baby eats more solids and takes in less milk, overall milk supply naturally decreases and you may see a decrease in pumping output. You may not notice a change in nursing pattern, as some babies nurse just as often, but take in less milk during those sessions. If baby started solids early (before around 6 months) or is eating lots of solids early on, you are more likely to notice a drop in supply. A very gradual start to solids around 6 months or later is less likely to affect milk supply.
I also know that you are dealing with a lot at the moment, and are quite stressed out as a result. This could also be affecting your supply. Whatever the reason is, relax, stay hydrated, eat well, and try pumping a bit more frequently and it should normalise
Hugs, hun