Thanks everyone for your support in this. Makes it less frustrating. I'm going back to get more blood tests today. One of my roommates' dad is a cardiologist and I mentioned this to him and he asked if the doctor measured my blood pressure when I stood up, like to see if my blood pressure drops suddenly when I get up because maybe that could be it. I might ask the doctor to do that. I feel weird suggesting things I think it might be (like pointing out that it might be blood sugar since I tend to feel worse if I've not eaten in a while or I eat too much sugar, or asking about the blood pressure thing) because it seems like he'd know this since he's trained and I'm not. At the same time, it would obviously be bad to just stay unhealthy because I didn't want to annoy him. Oh well, hopefully something will get figured out soon. Maybe if these tests don't do anything I'll mention his earlier suggestion of a CT scan to see if he still wants to do that but has forgotten or if he just doesn't think that's necessary.
It's the student health centre at my university so it's really busy and I think they're trying to move through us quickly, so then I wonder if maybe that's affecting how well he's thinking about it all. Like I looked up the tests I'm supposed to be doing this time, out of curiosity, and one of them mentioned that I shouldn't eat for several hours before taking it. He never even mentioned that to me or anything, so it sort of makes me think he's not hugely focused on this. But I can understand that since there are so many people to be trying to process. Oh well, hopefully we figure out the answer soon.
Thanks again to everyone who responded.