My 20 month old german shepherd has such a rapid heart beat compared to my other dogs what is the reason for this please we rescued her she was very malnourished & has not grown as she should have done is it due to this reason she loves running around the fields & playing which she does extremely well but she doesn't walk as well because her hind legs are too long

Puppies will have a very fast heart beat.. Normal for a puppy is around 220 beats per minute.

If he acts weird, pants excessively ( like he's out of breath or just ran a mile in 5 seconds) take him into the vets office immediately.

But if he's normal to you in every way possible, such as, eating and drinking, no vomiting or diarrhea or excessive salivation, no coughing or sneezing... Then his heart rate is normal. When he reaches adulthood, his heart will start to beat the normal 60-160 bpm

Have you had her checked by your veterinarian, not only as to heart rate, but rhythm, strength.
Whether she may have "bounding" pulses.
Does she "give out" quickly? And have you had her heartworm tested and vaccinated?
When are you checking this? While out playing? While at rest?
Being that she is a rescue, it is possible that her health has suffered from poor or no health care.
You can do quick test at home to see if she is getting the needed oxygen into her blood stream.
Lift her lip and look at here gum tissue, it should be nice and pink, if you press it with your finger it will blanche out to white, but should turn pink again in a matter of about 3 seconds when you release you finger.
Pale gums that do not refill quickly are a sure sign something is wrong.

Sorry about all the questions but more info helps us to help you.

Hi!
Thank you for that possibly worry more as she so badly neglected as a puppy & malnourished so didn't form properly either & now at 20 months thought her heart beat should have slowed down
Very best regards