My Dad experienced/is experiencing similar side effects.
I'll give you a low down on my family's GBM story.
My brother and I witnessed my dad's seizure, so that's how we found his tumor located in the right frontal lobe. 1 month later, Grandpa was having trouble finding the right words for things. My uncle did a CT scan looking for a stroke and found his GBM tumor in the left temporal lobe.
Both my dad and grandpa went through the craniotomy followed by 6 weeks of radiation and Temodar. Grandpa's follow-up scan was clear so he is doing Temodar 5 times every 4 weeks.
Dad's follow-up scan showed 'change,' which could have been dead tissue, but didn't know for sure so they are treating as if it progressed. Dad's treatment is now Avastin via infusion once every two weeks, Etoposide for 21 days, and cyclophosphamide 7 days on, 7 days off, then 7 days on.
The DR believes the fatigue, chemo brain are effects from treatment and not tumor growth.
Dad is also using the Novo-TTF device (Tumor Treating Fields). It is a new 4th treatment for GBM that has no side effects. All you do is have electrodes attached to your head to stop cancer cell division. Look it up. The science behind it is amazing. This DR is the only one in KC authorized to use the device within 150+ miles of KC. Dad is one of two patients here on the device, one of just over 100 in the US, and one of the few that is using it preventatively. The FDA approval came in April 2012 after studies on GBM recurrence showed to extend life expectancy.
The change in Dad is hard to adjust to, especially for my teenage brother. His logic seems to be off. Things like: Calling my grandparents and thinking they are on the computer because he got a busy signal even though they have had WIFI for 5+ years; having to remind him to put his seat belt on every time we get in the car; getting a thought in his head and obsessing over it until he can do something about it.
I find it hard to pin point what exactly causes this because there is so much that has changed for him.