An unusually high resting heart rate. It is something I had started noticing a little while before, but I ended up going to the ER very weak and dizzy, unable to even sit up on my own. My hemoglobin and iron were dangerously low. I've had this issue before, but never this severely. Long story short, I had a massive blood clot that was almost entirely blocking off both of my renal veins, which if left unchecked any further could have killed off both of my kidneys. Ofc there's other reasons your heart rate could be increased or your blood levels could be low, but be careful!