I get this too, I think it's DP as well. I feel exactly the same with the brain dead periods, it's a living nightmare. I think 'cause I can hardly do anything my brain really stagnates as well- I think that might contribute. The thing is our brain doesn't let us use our brain anymore!
Defnitely fight to get more physical check ups to be sure. Just don't go to a **** locum GP who knows nothing about MH to get them. I didn't even mention mental health to him, just tried to describe these symptoms, but as soon as I walked in he looked at me and I think he thought I was psychotic 'cause I was so out of it with the DP. He then decided that meant he could refuse me more physical check ups. Even though there was no mention of psychosis in my notes. ANyway, even if I had been, he had no right to refuse a physical health check-up. Search out a GP who is said to be decent or better still try to get your MH team to refer you if you have one.
But I think a lot of us are regularly convinced we have a brain tumor or something because of this...