What could cause my heel to be sometimes tight but always in pain?

On the bottom, . It is common to have plantar fasciitis; if on the back, it could be achilles tendonitis. Have it examined professionally. It can take a while to resolves completely, but there's no need to suffer longer.
Plantar fasciitis. It sounds like you are suffering from plantar fasciitis if the pain is in the bottom of the foot.
Plantar fasciitis. The most common cause of heel pain is plantar fasciitis: an inflammation of a thick band of tissue that helps hold up the arch of your foot. It attaches to your heel bone, and is usually the most painful for the first few steps in the morning, or after sitting a long time and then standing quickly.
Plantar fasciitis. Plantar fasciitis. A better diagnosis could be attained with more information such as when it hurts, what you do to make you feel better and what the foot looks like as far as swelling hot or cold, etc.