Maybe not your flat
The first joint (dp), or larger second (mcp)? Is is the pain located on the side, top, or bottom of the joint, if it is the first dp, it can be traumatic; it can be arthritis; psoriatic arthritis
loves this joint.
If on the bottom of the first, could be tendonous. If at the bottom of the mcp joint, you have two sesamoid bones that can be fractured, or inflammed.See a good specialist who can sort!