Could either be soft tissue or bone. Might get an x-ray to see if it's a bone spur--but if it comes and goes it's probably something soft tissue: then it could be a recurring bruise from the way you walk or shoes, or could be the pull of a tight tendon or muscle.
Are you active: do you walk or run or do other regular exercise (outside the home & kids, LOL!)? Is there anything you do that elicits the pain or any particular time it occurs?
Try the following: get some gel heel inserts for shoes (Dr. Shcholl's type, at the drugstore); that will also help if it's a bone spur.
If it's a tight calf muscle/tendon pulling and causing pain, try stretching gently, both the calf muscles and Achilles tendon: Stand about 2' from a wall with feet together; lean toward the wall keeping heels on the floor until you feel a stretch along the back of the calf. Hold (breathe!) for 10-30 seconds, then lean a bit further in to increase the stretch. Now bend your knees, still keeping heels on the floor (this shifts the stretch from calf muscle to achilles tendon).
Wear good flat supportive shoes (walking, running, or cross-training shoes) with good cushioning under the heel (chuck the flip-flops!).
Best of luck!