Help, my heart rate is 100bpm for no apparent reason, i have no pain or palpitations, but a history of health anxiety. using an iphone app to measure?

Many possibilities..: For a 20 y/o male, a resting hr of 100 is unusual. Many possibilities, most are benign. The phone app may be erroneous, you're checking only at times when you are anxious, or you are deconditioned (out of shape). I'd suggest regular daily, aerobic exercise, and if still fast after 3 or 4 weeks, then see a doctor. A 24 hour holter monitor would be appropriate as an initial step.