I have been treated for anxiety and depression which has nearly always manifested as health anxiety. I have made a personal decision to try to deal with this without medication since a little over a year ago.
I don't know a lot for sure, but I do know that I (and I suspect others with the same problem) have an exquisitely sensitive awareness of body states. Most people don't notice their heartbeats, but often I do. Most people don't notice twitches, but once it became an issue in my household, everyone was reporting twitches from time to time. Every twitch, buzz, pain, "weakness" and blip is going to somehow get past the filters that keep the rest of the world blissfully unaware of the bubbling, self-propelled meat soup their consciousnesses inhabit.
I have something called "mitral valve prolapse." It is like a mild heart murmur. Benign. Yet it is often associated with a syndrome that includes anxiety, fatigue and a lot of other things that one could not realistically link to the bare physical findings. One of the more intriguing ideas is that the little physical variation in the heart valve is reallly just a "marker" that suggests some widespread, subtle difference in the way some people are made, including the way their brains and nervous systems are "wired."
Before my PVCs were diagnosed, I was told I had "panic disorder." Yeah, you tend to panic when your heartrate goes up to 250

Now I'm on atenolol and don't have the problem anymore. So there was a physical component to my "psychological problem." Both were real, but the physical component was not as threatening as it seemed.
I have also read that in people with OCD, certain parts of the brain run "hotter" (as evidenced by increased blood flow on a PET scan). I am seeing this less as a "mental problem" or a "physical problem" as a "mind-body" problem.
Interestingly, once I decide I have another disease, I forget about the previous ones. I bet this is common. My brain doesn't care exactly what it has to freak out about. Anything will do. The cycle of awareness-anxiety-checking-reassurance is the same. There was a time I took my temperature mulitiple times an hour. Now I do a strength test. This is classic OCD, from my understanding.
I think I'm "wired different." I think there are things that cause real symptoms, like twitching and perceived weakness (really fatigue). Sitting at my desk all day, or driving are two things I KNOW will cause my to get more symptomatic. But, due to my different wiring, my brain does not filter these things out from my consciousness, but amplifies them and injects them right into the cortex.
Paradoxically, I have sometimes gotten relief from just saying "Okay, I give up. I accept the fact that I have [whatever]. Today, though, I can still walk around, so I'm not going to waste it." When I pay really close attention to what is going on, the distress I experience revolves around the symptoms themselves, and their import, the uncertainty that fear rushes to fill while masked as certainty of the worst. (I assume that, when our anxiety is at highest, all of us 100% positive of the worst. I'd be curious for feedback on that, because I think that sets one sort of problem off from another. It's natural to be scared about something, but a feeling of absolute certainty of the worst is a different animal, I think.)
I have been dealing with the current issue in one way or another for seven months. Most of the information I have read indicates I will be moving beyond the time period when I should be noticing something other than twitches and hard-to-pin-down sensations of fatigue or "weakness." At some point next year, I hope I'll my self-reassurance will carry enough "real world" weight to keep me from going to pieces from time to time about this. I suspect, though, that something else will come along.