Been playing upright for 7 months now… I had the exact same problem a couple months back.
1) rest for a couple days. seriously.
Once it feels ok and you start back in...
2) left hand shape. Move your thumb DOWN when playing so it's more behind your 2nd finger (almost behind your 3rd finger but not actually). This tiny change was HUGE for me. Gives your 3rd and 4th finger more leverage.
3) don't press so hard for a while. think about the weight of the bass pressing into your hand. watch this vid… especially the bit starting at 4:20 if you want to skip: http://www.youtube.com/watch?v=aiJCOWAhE9Y
(but watch the whole vid, and ALL of his vids)
3) vomit exercises. 3rd and 4th finger. (I do one octave for 4th and two octaves for 3rd to avoid playing too high with 4th finger. I don't play arco yet… so I do vomits by plucking steady 8th notes on right hand) Vomits really helped
me slowly and safely build strength in 3rd and 4th fingers. Absolute minimum pressure on the string needed to produce the tone. But STOP the split second you feel any pain. Do not play through the pain in this situation.
4) Cut back your fingernails (even more) and lotion your calluses
. I'd NEVER used lotion on calluses before, but upright teacher instructed me to do so, and it creates a softer pad (but still strong) to help cushion your finger a bit.
5) Did I mention rest for a couple days?