Complete change in behaviour! Please Help!

Hi Guys

I have an issue which has really gotten me at my witts end! I have a 3 year old gelding that I purchased about 4 months ago. I have had absolutely no trouble with him since I've had him apart from a bit of dominance issues when it came to feed time but I quickly squashed that, but in the terms of riding and his behaviour he was an absolute pleasure! That is until about a 3 weeks ago I got really sick and wasnt able to ride for about two weeks and during this time I had a family member taking care of him, but he can't ride so my boy wasnt being ridden at all. Now when it comes to riding him I'm running into all sorts of trouble, he is forever trying to bite me when doing the girth up (which he hasnt ever done before) and whilst riding if I put pressure on my inside leg he swings his head around and tried to bite it (but not the outside!) and he is now swinging his head in the air and tossing the bit around in his mouth which is also something that he had never done before, if I try to break him into a trot he'll pig root and the same as a canter at first I thought maybe its my saddle so I tied riding in a different saddle but no change. Then I thought maybe he is sore but I have seem him on numerous occasions galloping around bucking in his paddock so that seems to make me think that it isnt that. So my question is this really, could he be sore and my weight in the saddle is making it painful enough for him to try and bite me or do I just have a three year old that has been out of work and is now trying to put it all over me???

Any suggestions or advice would be muchly appreciated, I'm just at such a loss with this because prior to the two weeks off he was practically the best horse I had ever owned and was a pleasure to ride.

It's possible that he is out of alignment, due to overactive play, and he has rib out, as they say. Maybe have him looked at by a horse chiropractor. He's not grumpy in other ways, but only when girthing? So is it more on the left than the right side?
Is he grumpy if you ask him to canter in the round pen or on the lunge line?

Maybe this is totally out of line, but is it possible that someone rode him without telling you?

Or worse abused him while you were not well

Sounds to me like he's in pain. Try running your hands down his back and around the girth area. Watch for him to give or react in some way to the pressure. Could be his back is out, pulled muscle, cyst, etc.

Also possible he's having a tooth issue (wolf tooth coming in?) and knows you are going to hurt his mouth with a bit so doesn't want to be saddled.