What Causes Blisters On The Bottom Of Foot?

Hi I began noticing a few blisters on the bottom of my right foot... As they hardened. Now it s like thick hard skin... But I have more and more now and three of the little bumps of skin have developed black spots underneath them... At first I thought they were just dirt or black cloth from my sock but I looked at them closer and found it s beneath my skin. I think I have some foot fungus and I am scared. Please help me with any advice. Thank you.