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is a possibility, but why should one be stressed while sitting only?
of 87 is not really fast - 60 to 100 is in range.
Probably, one is only aware in particular situations / position.
If concerned, it has to be documented by 24-hour ambulatory monitor (Holter) - taking care to note down the posture. If such a relation is actually confirmed, further assessment and assistance from a cardiologist will be ideal
Wishing speedy recovery