I noticed it early this year. It was sore and I could feel the lump easily.
The lump is actually on the tendon running from the heel up in behind the ankle. When I dip/wiggle the foot holding the tendon the lump is actually on the tendon.
I seem to get a 'spikeing' pain at the lump and likewise on the other foot as well at the tendon but there isn't a lump on the right one.
I did have a severe ankle sprain in the late '80's for the left leg. Leg was in plaster for 10 weeks. I could see the little dots of bone on the xray pulled up and off the foot at three places. No broken bones as such.
Don't have diabetes or thyroid that I know of. My younger sister has to take tablets for an inactive thyroid, I think. I am 5ft 6 inches tall and weigh 105kg.