Why is the skin thickening under my toenails?

Could be a fungus: But without seeing you or taking a culture, it's impossible to say. A podiatrist (in person) would be able to give you the answers you need.

A few things: Skin thickening under the nails is not unusual if the overlying nail is thick and/or deformed (such as from prior injury or fungal infection). An underlying bone spur on the toe can cause callusing under the nail bed. See a podiatrist for a proper diagnosis and get appropriate care options.