YES. you cold be having a retinal detachment. you need a dilated eye examination right away. you know most of the posts are are not emergent...but this one could be.
any 'ocular trauma' needs a dilated eye exam. especially ocular trauma with symptoms as you describe. go to an eye doctor (optometrist or ophthalmologist) ASAP and tell them your story. they will fit you into their schedule and have you examined today to rule out retinal detachment. the sand is the least of your worries. it is within the realm of possibility that your 'black spot' is a retinal detachment or hole or tear that may need surgical repair. if that is the case then time is of the essence.
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"cold" should have been "could",
"you know most of the posts are are not emergent" should have read "you know most of the posts HERE are not emergent"