What's actually causing it
Your plantar fascia is the thick band of tissue running from your heel bone to your toes. Plantar fasciitis is soreness and inflammation of that band, so the pain sits under the heel, sometimes dull, sometimes sharp 1.
A better question than "what's wrong with my foot" is "what changed." Plantar fasciitis is more likely with running, dancing, aerobics and jumping, and more likely again if you've increased those activities or started a new form of exercise 1. A change in footwear does it. So do shoes with poor arch support, tight or weak calf muscles, flat feet or high arches, being middle-aged or older, carrying extra weight or being pregnant, and time on your feet 1.
That last one matters more around here than it might elsewhere: retail, warehouse and trades work all mean hours on hard flooring, and a roster change can shift your standing load more than any training program would.
Worth knowing early: walking differently to avoid the sore spot can cause other problems and slow your recovery 1.