Leg weakness is not typically a first sign in PD. However, PD usually is associated with stiffness and slowness of the limbs, and this may feel like weakness. If you have a tremor, especially if it is in the leg, I would be more likely to think about PD as a cause.
Other things to consider:
Pain can lead to favoring the other leg - and a sense of weakness.
A muscle or nerve problem in that leg.
A back injury causing pain in the leg or pinching of a nerve to the leg.
A stroke - but this is not likely, given that it is quite sudden in onset, and you would have gone to the hospital, not WebMD.
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