I have had the same problem in the past. And when I saw someone about it, they said this can be caused by a weakness in the fingers. Here's a good exercise to strenghten your fingers...

play a bar chord arpeggio'd (spell check) like this, using only 1 finger. Make sure holding down all the strings so they're not muted. Then do this with all your finger.

E--1--2--3--4------------------
B--1--2--3--4------------------
G--1--2--3--4------------------
D--1--2--3--4------------------
A--1--2--3--4------------------
E--1--2--3--4------------------

Do this a couple of times with each finger. this technique will strenghten your fingers, but the best is to buy one of those gripmasters. they work well. Ben-say or any other muscle relaxer works too.

If you are still having problems, or it hurts to do this exercise. I would take to a doctor about it.