For the second time, I happened to start to get a dizzy spell (without bending or getting up or anything I could point to) while being near to Eric’s blood pressure cuff, so I slipped it on quickly. Like last time, my blood pressure was normal (this time it was 121/72), but my pulse was very low for me. Last time, it was 52; this time, it was 44. Last time, Eric insisted that it was probably a fluke or a battery thing, even though the blood pressure numbers looked fine; when I took it again a few minutes later, it was in the 60s. This time, I decided to go consult Dr. Google.
I’m actually reassured. Lots of articles are saying that athletes often have low pulses, and they can cause dizzy spells when changing position because it takes even longer for pooled blood to circulate to the rest of the body than it does for people with faster pulses. I don’t know how much of an athlete I consider myself to be, but my blood pressure has always been good, even before I started running, and my heartrate is very fast to recover to resting rates after I run, which is purportedly a good thing. Maybe that’s all it is? The articles just say to go slowly when changing position, which I’ve been trying to do. They also advise to check with a doctor anyway, which I’ll do.
If I had to have something in common with the president, this is a good choice, I guess. 
Your mention of low heart rate made me think of my mom. She has extremely low heart rates and she isn’t athletic at all. hers is caused from her thyroid. I googled it real quick to see if I was remembering it right and a quick glance says I am. I don’t know if you have had your’s check or not but seems like something worth looking into at your doc appt. Especially since I belive it is just a simple blood test.
My husband has a low heart rate and he is a big time body builder. He suffers, occasionally, from the same dizzy spells/headaches that you are describing. I would not at all be surprised if this is the cause of your spells. Also, the thyroid theory is very possible (hubby that as well). I wouldn’t worry too much, but your appointment is a good idea.
I have low BP. Make sure you are drinking enough water and eating enough throughout the day!