I had high blood pressure for over twenty years but since it was without other symptoms I was told by doctors to just watch my salt intake and lose weight.
Two years I gave up caffeine and my blood pressure has remained around 111/72 up or down with 10 pts. I have had my blood pressure as low as yours and I would attribute mine to significantly less sodium in my diet at the time.
On the other hand, I also have noticed that when I eat at an Asian restaurant or Asian restauarant prepared food, it jumps up 145/90! Also, when I eat delivery pizza like from one of the national pizza chains, my blood pressure skyrockets!! Within eight hours it goes back down but it is something that I make note of. Lots of water will flush away the extra salt in my system and everything returns to normal. It is a very real reminder of how close high blood pressure is to the types of foods we eat.
Initially, when my blood pressure was low I experienced fatigue and sometimes lightheadedness. I was instructed to have a tsp of salt (yuck!) to bring it back. Instead, I usually have 4 oz of Gatorade. I now keep Gatorade on hand for such times. You might be losing electrolytes along with the water weight you are losing. I have found that has corrected the situation.
My husband typically has a pulse that is as low as yours and has for years. Sometimes, he complains of fatigue as well. Both of my parents see doctors regarding heart medicine and my mom says her doctor constantly wants those "numbers" lower and lower.
I also take magnesium as well. I do not eat bananas very often because they also lower your blood pressure so if you are you might want to cut back to less or none. Your heart needs a balance of magnesium, calcium and potassium for it to work as it should. Since, we are dieting, we are probably not getting all the minerals and vitamins we need, since we are typically eating less food. Have you tried specific supplements, I use herbal, and/or a basic multi vitamin??
I wouldn't worry though. I think your body is just adapting to this new statis and it is getting used to not having a lot of junk in it. It will stabilize in a short while but those things I mentioned have worked for me. I now have a lower blood pressure and I have energy to do all I need to do each day.