Dr. Spector, the other oncologist (in the same office as Dr. Porubcin), said that maybe what is happening to mom has something to do with the clots in her lungs breaking away a little piece at a time and going to her head. Even though she has the filter in place now to prevent clots from reaching her lungs, she still has clots in her lungs and those are dissipating into her blood stream. Some smaller ones can still make their way through her system before they dissolve, so that may be what is happening with the pressure in her head... it's a clot.
The clots will dissolve in time. Mom just has to make it through until they do. There's nothing to do but wait... all the other preventative measures are in place. She got a prescription to help alleviate the anxiety when the symptoms start up, so hopefully that will help.