Brief Answer:
?? Probably a blister (friction wound), cyst or hematoma

Detailed Answer:
Hello ma'am and welcome.

Thank you for writing to us.

I have gone through your query with diligence and would like you to know that I am here to help. I somehow doubt it is a hard bulging vein, and suspect it to be:

+A pocket of fluid or air (probably a blister): Very common in the region due to friction wounds (from uncomfortable footwear)
+Hematoma: Also possible from uncomfortable footwear, but also possible due to injuries
+Cyst: Unknown cause, but harmless

I cannot comment accurately on the most likely cause because I would have to check on the consistency, mobility, and a blood panel to confirm the cause. But it does not look like something to cause concern, and should in my opinion also be self-resolving (meaning not requiring any medical intervention). But if it lasts for more than a week without showing any signs of regression, then you can visit your doctor for some help.

In the meantime, avoid tight and uncomfortable footwear and just leave the region open and avoid any extreme temperature compressions.

I hope you find my response helpful. Please do not hesitate to write back to me for any further clarifications, I am always here to help.

Best wishes.

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