What causes low blood pressure and high pulse rate after exercising?

Dehydration. This is why you need to hydrate yourself adequately before, during and after exercising. Especially if you are exercising in a hot environment. See if this improves with adequate hydration (water and/or electrolyte replenishment drinks). If not, then schedule a visit with a Cardiologist for an evaluation.
post exercise. Post exercise, the cardiac output drops fairly rapidly and the peripheral resistance less rapidly increases, this can cause lower BP and higher heart rate.