What's wrong with my Foot?

i hurt the back of my foot a few months ago, it swelled and i couldn't do any physical activities for over a month. The day i hurt it, i was running in PE, i had to limp on it because i couldn't walk on it without pain shooting up the back of my leg. I didn't pull a ligamente neither did i hear the bone at the back of my foot crack. Luckily it was the school holidays in a few days so i got to rest it for a week or so. I got out of P.E for a month but had to return because i was missing out on too much, my foot was still swelled up. A week or two after returning to P.E, i hurt it again and couldn't do any sports. The next day i was walking to the bus, with a friend, when i heard a slight click and pain again shot up my leg, i thought nothing of it. That night when inspecting my foot i found a small swelled and red lump which was tender but not too bad. My parents made nothing of it so i didn't either. Then another couple of days passed and i inspected my foot again which still hurt to walk on and was still a little swelled and i found that the lump had grown. It was a hard lump. It stopped hurting after a few weeks. However i couldn't run properly on it. Then i went out for the day with a friend wearing flat shoes, and that night after again inspecting the hard lump, it almost felt like gristle, however in a few hours it had turned hard again, and hurt. What is wrong with my foot?