Ok, so I'm playing "Lonesome Tears" by Beck. The chords go:
C#, B, F#, A (barred at 5th)

There are a couple other variations on that, but my question is this... after a few times through, the muscle between my thumb and forefinger starts to bulge and feel like its going to cramp. I'm trying to keep my thumb opposite my middle finger on the fretboard (I think this is right). I'm just wondering is this something normal that you've got to build up, or maybe there is something I could be doing wrong (I know its hard to say without being able to see my hand)? Maybe there are tips someone could offer?

The 5th string root major barres, in particular, really kick my ass for some reason.

Thanks =]
Without seeing your grip it's hard offer advice on technique. However, most beginners need to build hand strength. Strength in the hand improves in two ways, through better muscle tone and muscle coordination. More practice will improve both points, it just takes time.
You don't have to keep your thumb in any particular area, just keep it where it's comfortable. This might be your problem. Also try to keep your hand relaxed as much as possible. You don't want to get all tensed up and have your hand cramp. I find that I sometimes tense my hand up with barre chords, maybe you do the same thing...?
Hey! Yeah strength takes time, I tried the way I think you hold your hand, but for me, thumb behind index fingers seems more natural. If you wanna train this. Play the song till you start feeling the pain (DONT KEEP PLAYING AFTER IT, haha). And see if you can keep playing a bit longer the next day. It's like running or any sport. I mean, you can't run the marathon when you start running either right? Good luck man!
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