My mothers heart rate is very fast. its about 90-110bpm. even when her blood pressure is normal her heart beat is fast. what could be cause. age 50?

Increase heart rate: Can be simply due to lack of physical conditioning, obesity, drug induced(to much caffeine), anxiety, hyperthyroidism, etc. I will recommend a good history and physical by a primary care physician.

Rapid heart rate: She may have an arrhythmia, which can be secondary to an underlying cardiac condition, she needs to see a doctor and have an ekg to start.