I suddenly got a deep reddish purple round stain under the skin of my right pinkie finger, near the last joint that bends by the nail. its opposite the nail. though i had no injury, i'm worried that its a blood clot.

Sprained finger?: Perhaps you twisted it without realizing it? If you don't have bruising or stains like that elsewhere, then watch it to see if it gets worse. It's an odd place for a blood clot without trauma. If other purple stains or this one painful or worsening, don't ignore it.

Purpura: Call your pcp, it could be a splinter hemmorhage, an infection or trauma.

Skin lesion: Best to have someone look at it, either a dermatologist or hand surgeon. It may be ok to observe it, but may possibly need some other treatment including biopsy. Good luck.