I second Maris's comments about the filter causing problems as well. Where I used to work, they made us wear respirators when making slide preps of hairs using a toluene based media. Despite having it professionally fitted, I had problems breathing with it. I have asthma and I've heard this is common with masks.
If you insist on using one, one of the ones covering acids is a better bet than the one for organics you linked, imo. If you mixed anything from a powder that day, that is likely the cause as well. Powders are much more easily inhaled in quantities that will cause problems (the main reason I use things that come as liquids). A particulate filter should be fine for that. You also need to be careful to make sure you don't get a cloud of powder flying up when you dump it into the container.
If you do your tray work by time rather than by watching (better for consistency), you only need to be an arm's length from them to agitate the tray. Standing over them won't be good for the lungs or the back.