I am a 35 yr od man in great shape, work out every day free weights, aerobic activity, basketball on Monday nights. today I ran on a tread mill I ran hard 8 mph, my heart started to ache, I slowed down, pain went away, then went back to 8mph. and finiched 4 miles...this has happended in the past when I start back up running...but it goes away...what could this be?