something's wrong with me

I have an appt with a sports medicine doctor on Wednesday because I'm completely baffled by this. I've been exercising for a long time and have never felt anything like this. 

Went out to do my birthday mile -- made it to 600m at 6' pace then had to stop and was doubled over in tremendous pain: stomach, legs, can't really describe it. Had to walk the mile back to my bro's house.  

Strength and anaerobic stuff not affected. Hope it's nothing serious... 

Kind of bummed about this so I won't be posting on here for a while. So confusing.