Quick Update and Question

I'm happy to say that the second graft my surgeon put in is working really great. This last week was the first with both needles, 15 gauge. The PA at my clinic had them do an ultrasound to check my flow, and the two results were 500 and 510. He said with grafts they like around 800, but considering before the first graft surgery my flow in my natural fistula was 75, I think I'm doing better! And, it appears that finally I will be getting my catheter out this coming Friday! I will be talking with the case nurse at the VA Monday to confirm.
At my clinic, they use Fresenius 2800H machines. I have never had a full-blown "crash"(fainting,etc.), but last Saturday, after taking about four steps out the door into the lobby after my treatment, I froze in place as the room started to "spin" and I was severely lightheaded. Noticing an empty chair to my right, I managed to sort of throw myself to it and support myself on the chair's arms. Bent over slightly, forced myself to breathe slowly, and the dizziness passed after a few moments. Not fun! Strange thing was my B.P. readings were OK, and I was feeling fine when I started out.
Not wanting to go through this again, I have begun to closely watch what my after-treatment weight has been. Last Saturday, I had seen that my weight was .5kg under the dry weight set by the PA. Last Tuesday, my weight was .4-.6kg under the dry weight.
When I asked the nurse about this, she said that the machines can sometimes do this--no big deal. I was amazed at this. I know that since they don't use heparin on me anymore, they have to do a "flush" of my lines every 30 minutes, which they have to figure into their setting of the machine, but how can the machine take off more--sometimes way more--than what it is set to take off?
Does this make sense? I don't really believe the nurse, and plan to talk to the clinic's head nurse this next week.
So, glad the graft is working, and am looking forward to getting rid of that cath. this week!
Jon