The aspirin may be causing her blood to be thin enough to increase the swelling. You can read about aspirin use in dogs here
It is possible she has a liver or spleen problem causing the abdomen to swell. You can read about spleen issues here
and liver problems here
It might also be her heart is not working as well as it used to meaning that the fluids are not getting to the kidneys the way they should to be excreted from the system.
It could be that a diuretic or a heart med or both would make her feel better.
I think its worth a general check up to see if there is something that can be done or if she has just reached an age where her body is not going to function right anymore.
Hope this helps you!
I am so sorry for your loss.
If you have not seen www.petloss.com and the rainbow bridge story you may find some comfort there.