Sounds like dehydration to me.You can go years and have no problem,then wham!it gets you.It made a believer out of me.Had a friend that does a lot of triathlons,never had a problem,then one day they only had 4 bottles on a 5 hour ride.Next thing they remembered was waking up in the hospital.The pro's describe stories like yours also.Remember,it can take days or even weeks to recover from dehydration,depending on how dehydrated you were