You probably nicked if it bled like b+ggery the arteria princeps pollicis - main artery to the thumb.
This is what Henry Gray had to say about it:
"The arteria princeps pollicis arises from the radial just as it turns medialward to the deep part of the hand; it descends between the first Interosseous dorsalis and Adductor pollicis obliquus, along the ulnar side of the metacarpal bone of the thumb to the base of the first phalanx, where it lies beneath the tendon of the Flexor pollicis longus and divides into two branches. These make their appearance between the medial and lateral insertions of the Adductor pollicis obliquus, and run along the sides of the thumb, forming on the volar surface of the last phalanx an arch, from which branches are distributed to the integument and subcutaneous tissue of the thumb. "
The adductor pollicis has two heads - and you lack one, so in your case, it doesnt descend as it does in the script but remains under the skin which is how you could nick it.
Should be the same on the other side - you should be able to feel the artery close to the skin.
PP
DOI same variation myself - quite common apparently
well you did ask