Podiatrist, Dr. Rami Calis explains that during the summer people tend to be more active. Unfortunately, the shoes that go along with a lot of the summer activity tends to be less supportive, so people become more prone to injuries.
These injuries could be heel pain on the bottom or the back of the heel. Calis suggests four features of shoes that you should consider having to prevent these aches.
The first is to have something around the ankle to hold the shoe in place. Two, have some sort of arch in the shoe if possible. Three would be a thicker rubber sole to give you some shock absorption and four to have a slightly deeper heel cup to hold your heel in position.
Unfortunately many summer shoes don't have these qualities, especially flip-flops.
Dr. Calis says flip-flops are o.k. for lounging around. But if you're walking, he recommends a shoe with more support.