High hr day after surgery. activity has been limited due to pain but no other symptoms other than hr in 120s resting. cant feel it beating fast. is this tachycardia likely due to extreme sleep deprivation and pain or blood clot?

You don't mention what type of surgery you had. I assume you're not running a fever which can raise your resting heart rate. 120 is a rapid resting heart rate and warrants an explanation,examination &some testing like EKG,cbc&diff,metabolic profile,thyroid testing,urinalysis. I think you should notify your surgeon of your tachycardia if you're not hospitalized.

All of the above and more can contribute to tachycardia. At a minimum an effort should be made to address the pain better, improve sleep as well.