I have very flat feet and i wore some heels to church on sunday, now there is like a tender mound of flesh on the top of my left foot, why is that?

Impingement: If your feet are very flat, you may have some impingement on the soft tissues on the outside/top of your foot (called the sinus tarsi). The high heels force you to plantarflex your ankle, making everything less stable. This may have caused further impingement on those tissues in your sinus tarsi, making it more inflamed and painful. See a podiatrist for a full examination, diagnosis, & solution.

See a doctor: It is hard to diagnose this problem without an exam. I suggest you see a podiatrist or orthopedic surgeon. One common diagnosis is a ganglionic cyst which can occur suddenly. It can increase and decrease in size over time. There are other types of soft tissue masses that can occcur in that area so a trip to the doctor is warranted.

Flatfeet: Your flat feet are not the type of feet designed for those heels you wore on sunday. Because of your foot type, it probably was bunched up and swollen in the heels for a longer period of time than preferable. I understand it's embarassing to remove off shoes in church, but it may be best not to wear those heels anymore due to the pain they cause. Best of luck.