Aching like cramp or aching like strain/sprain or aching like sore muscle? Is there throbbing, swelling, shooting pain ever involved. When you walk do you limp? When you walk do you feel anything out of the ordinary?
Do you warm up? Does it hurt before your warm up? How about after? Do you cool down?
Does it get better or worse or stay the same as you continue running? How about a few hours post exercise?
How is your range of motion? Can you move normally? How about when you stretch?
How about your shoes? Are they in good condition? Especially inspect the heel region. How about your running surface?
If it's just a low level ache that pops up at the end of a long run it might be reasonable to do a good warm up, stick to favorable running surfaces, reduce distance/intensity, do a good cool down and see how things go.
If you're limping, swelling, and having shooting pains it's time to seek medical attention.
Somewhere in the middle? Maybe you want to switch to biking or rowing or swimming for a week then ease back into things. Variety is good for the body and mind.
Good luck