I have similar trouble. Part of the problem might be since they just moved. A lot of dust gets kicked up during the moving process. Also, it it's an apartment, the landlord may have just had the carpets cleaned (those chemicals set me off) and the walls touched up with paint. Things might be better in a couple months, just encourage them to keep up with the vacuuming! It could also be something in the bathroom where moisture loves to cause problems.
I have trouble breathing even in areas where I previously had no trouble. I've gotten used to certain environments and when I'm not in a familiar area, my lungs and allergies are a dead giveaway. Even the slightest change in air quality (for example going to a coastal area to one more inland) causes problems for me.