My first question is, which side of your index finger? I’m trying to think through this as a player, pretty familiar with anatomical structure and how to use the body efficiently, without doing harm. This is the whole reason why, if you want to play all your life, it’s critical to set up proper hand positions and motions. That said, I’m wondering if you squeeze the neck at all while playing? Try playing with the thumb not touching the neck. This will be ‘yucky’ uncomfortable but you should still be able to apply about 2/3 of the fingertip pressure needed to get a decent sound. Pull the entire hand down with the fingertip on the string, using the heaviness of your arm, and gravity, to get weight into the string. No squeezing allowed.
It seems like just a million things could cause this, half of them unrelated to violin playing. But it would be good to make sure you are set up with a well adjusted violin posture, shoulder rest, head position, practice stance whether standing or sitting, attention to spine alignment, adequate stretches pre-practice as warm ups, and then violin elbow position and exactly when does the numbness start? Does it ever go away? Does anything else you do in life cause it to come back? Do you bite your nails or cut them too short or are you not cutting them short enough? This could effect the finger’s approach to the string and effect the angle. Also make sure your shoulder (left) is relaxed at all times while playing. No tension allowed, so look for that. Also do you write with that hand?
Just all kinds of questions before getting to the bottom of this but I am happy to think it through if you need a brain to pick.
This article says finger numbness can be caused by pressing too hard on the fingerboard and gives some feedback from players who have experienced this. Do you play guitar, too? You have to press much much harder on guitar than on violin. You also need to examine the position of your wrist as it could be nerve compression of some kind. Search that page for Julie Donnelly’s post in which she states:
“The most common problem here is repetitive strain injury of any of the muscles that cross over the median nerve, from the neck to the fingers. As violinists, every possible muscle is being contracted for hours on end. This causes an impingement on the median nerve, which will cause the fingers to go numb. Plus, the contraction in the flexor and extensor muscles, as well as the thumb muscle (opponens pollicis) will not only cause numbness, but pain at the wrist.”