For your age and your level of activity, it certainly could be normal, although it is getting on the low end of normal. Most of us have resting heart rates anywhere between 60 to 100 BPM. The SA node, the inherent natural pacemaking site in the heart, located in the right atrium
, inherenty fires at rates around 80 to 100 BPM. Most of us are a little bit under some parasympathetic tone, so our resting heart rates are most often under 80.
Athletes, especialy those that do a lot of aerobic activity, have a much higher degree of parasympathetic activity, therefore, their heart rate slows down to adjust for their cardiac output.
Cardiac output is equal to Heart Rate X Stroke
Volume or CO=HR X SV.
Since your heart muscle has grown and the stroke volume (the amount of blood that the left ventricle ejects from each beat)is much higher, than, if you maintained the same higher heart rate, then your cardiac output would become abnormally high, therefore, your body simply lowers one side of the equation and that is your heart rate. It's simple math.
I do not agree with the statement made "that anything that is below 60 is questionable". Thousands, if not millions of fit people have resting heat rates below 60. This is simply a sign of excellent cardiovascular health and a very aerobically fit heart.
Also, again contrary to an earlier statement made, blood pressures of 100/60 or even a little lower, while at rest is perfectly normal for people who are very aerobically fit. No cause for alarm there. I have had friends in their 20's who had resting blood pressures of 90/60 with absolutely no symptoms and nothing to worry about. They were just superbly fit.
What is worrisome in your case is the symptoms you are having. Could be many things. The flu
, mono, medication you could be on such as Beta Blockers, undiagnosed Type 1 Diabetes
. On the more rare side of the spectrum, it could be a cardiac problem, most likely an electrical one.
Or as many have mentioned, you simply could be overtraining, especially, if you recently started to train and have driven the intensity up too fast. Our bodies like to be eased into things and going from 0 to 100 overnight is asking for trouble.
Take a rest for a few days, if the problem persists or symptoms worsen, then see your doctor asap.