As a place to start, I would go to see a doctor who understands how to treat athletes.
To learn more about heart rate on the run zone, you most likely want to search the words heart rate monitor rather than just heart rate.
For example, from 2011:
Glad you have you post here!
Typically when your heart rate doesn't go up to where it should be the cause is related to glycogen depletion. Usually this happens when you combine hard training with either high volume or hot and humid weather training. When the heat index is high, you burn more "sugar," as you run along. Since you elevated mileage recently, your "tank" is low. The same thing happens in the Tour De France during years when it's hot and humid. Riders will see their heart rates at 120 beats per minute, yet if they let it go up to the 130's they feel like they are riding at tempo effort, not an easy effort, and if they hit 150's they feel like it's darn hard, and once they hit 165-170 they are nearly flat-out (not much left). That's not normal, of course, but they are so glycogen depleted (an hormonally depleted) that their true maximum heart rate has dropped significantly, and so the values associated with specific percentages are lower too. A 150 heart rate, for example, which is normally about 75% of maximum and a comfortable pace that they can ride for several hours, becomes 85% of maximum because their peak HR is suppressed. The same thing happens to runners.
HR is variable; it has different meanings for different days, and even the time of day. For example, at 6 am a HR of 140 may feel like a good effort (not easy, but not quite tempo), but at 5pm a 140 HR feels like you aren't working - it's like a jog. Another situation is when you consume a bunch of carbs in a 2-3 day period and you reduce training; your peak HR goes up and every value associated with specific percentages goes up. I no longer run - bad hips - but I bike a lot in my garage. I use a power meter trainer and my HR monitor. I can go out there at 6 am and do a VO2 max test (well, a 6 to 7 minute test flat-out) and hit no more than 169-171 HR. Another time of the day, say 4-5 pm, I can hit 180-182 HR while doing the same type of test.
My suggestions to you: First, use effort and pace more than heart-rate. HR should be a support tool. If you can't raise your heart rate but you push the pace a little and it feels hard, then you are over-trained and need to back off; jog a couple days and consume a lot of carbs. If you can easily raise your HR and it keeps going up easily, then you may be dehydrated. I can tell you this: A low heart rate is often more of a problem than a high heart rate. Yet, in all cases, you must adjust to feel. However, if you know what the proper pace is supposed to be (adjusting for weather, terrain, and time of day), then your effort and heart rate should fall in line.
You might take a look at my Heat Index Chart in the Tinman Charts section on the home page of The Run Zone. If you sign up (it's free), then you can see the charts. You'll be able to adjust paces according to heat index, and that way you won't overtrain.