Your blood pressure will naturally drop after a hard ride, because during hard exercise you stretch your blood vessels.
You might check your blood pressure after riding. Low blood pressure will cause dizziness when standing.
Those of us on BP meds, particularly Beta Blockers, frequently have dizziness when we stand - it is even discussed in the drug literature as one of the side effects of the drug.
I know my BP drops about 15 points after a hard ride, because I check it frequently.