I'm sure this question has been asked a million of times, but I couldn't exactly find the answer I was looking for...

So I decided to start playing guitar again (after I noticed that I'd let my talent slip). I noticed yesterday that when I was playing, the muscle below my thumb in my right hand (My fretting hand) began to hurt severly. I have never had this problem before, but these are my 3 possible explanations...

I'm not using a proper right handed fretting technique
I'm trying to do all the advanced techniques I could do before without warming up
I lack practice

I think it's my fretting hand technique...but I've been watching other guitarist play, and they do just as I do. One thing I noticed though is some players will let their thumb slide behind the neck about half-way, but when I try that, it feels awkward and I can't play. What should I do?
Warm up.

If it still persists after warming up, then you should fix up your technique. Having your thumb on top of the neck all the time will probably hurt it eventually.

Try putting your guitar higher on your strap.
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