Thanks for posting your query.
It is very difficult for you as a layman to compare and diagnose disease just by seeing photos.
The eruption as per your description seems to be clusters of papilloma i.e. warts
But they may be neurofibromas or callosities if a strap of sandal rubs that area.
A close shot photograph can be helpful or review with a dermatologist
would be helpful.
Hope I have answered your query.
If you have any questions I will be happy to answer.
Please close the query if you have no other questions.
Wish you a good health.