Welcome to HCM,
It is good that you are physically fit, regular exercise is good,sudden undue exercise may have caused vasodialatation and dehydration
which is possible cause, but you do not have any symptom to suggest ;low Blood pressure just to make sure recheck in both arms as there is some difference in two arms.
Take plenty of fluid and salt which will bring BP to normal range.
If it is possible get ECG done just to rule out any cardiac issues.
If your normal BP is not achieved and you have any thing to suggest there may be problem please go to your Physician and full examination and if required investigation for cause.
Hope you will recover soon.