Definitely see your doc. He or she will probably send you to a cardiologist for a checkup. If the findings are negative they will probably tell you that you have orthostatic hypotension (drop in blodd pressure on sudden movement). I have had it for years. I have to be careful when riding - particularly in warm weather. When it hits, you want to get prone with your legs above your head and it will clear up quickly. My niece the sports med doc tells me the standard drill is key: eat before riding, hydrate, keep up the electrolytes, eat periodically whilel riding and cool down with a lot of slow riding before dismounting. The other thing to watch for in us oldsters is meds. beta blockers are well know blood pressure droppers. Flomax can also be a culprit. I had to switch from Flomax to Avodart or I wouldn't be riding.