Limerence [from Wikipedia]
Limerence can often be what is meant when one expresses "having a crush" on (or infatuation with) someone else although limerence, unlike a crush, can last months, years or even a lifetime. It is characterized by intrusive thinking and pronounced sensitivity to external events that reflect the disposition of the limerent object towards the individual. It can be experienced as intense joy or as extreme despair, depending on whether the feelings are reciprocated.
The physiological correlations of limerence are heart palpitations, trembling, pallor, flushing, pupil dilation and general weakness. Awkwardness, stammering, shyness, and confusion predominate at the behavioral level. There is apprehension, nervousness, and anxiety due to terrible worry that any action may bring about disaster.