Can low blood pressure be caused by over-exercising?

No: It can reduce your blood pressure but it shouldn't drop it abnormally low.

Not usually: Unless there is valvular heart disease or hypertrophic cardiomyopathy. BP will increase normally during exercise but help to improve BP over the long run.

Low BP and exercise?: If you're exercising and not rehydrating yourself through drinking water and/or electrolyte-replenishment drinks, then I would think in the short term that your BP could drop while exercising.