It could be a bone spur, especially if you have had Achilles tendinitis.
I cannot confirm a diagnosis without examination,but it can be foot blisters or fibromas.
When a foot is hot and sweaty, the sock sticks to the foot. The sock and foot then rub against each other and the inside of the shoe causing blisters to appear.Plantar fibromas are firm, nodular masses which occur as a single mass or in clusters on the arches of the foot.
Wear shoes that fit properly and keep feet as dry as possible.Wearing wet shoes, boots and socks will increase your chance of developing blisters.So change socks regularly.
And use foot powder to help keep your feet dry.If the blisters still persist then get it evaluated from a dermatologist/podiatrist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.