I think most still use a sling rather than harness.
Check your posture, play whilst watching yourself in a mirror. Try and look side-on too (not easy because whilst trying to look sideways you will naturally move out of your 'playing position' a bit). Have a go though, and check you're not stooping down or bending to one side or other, or some similar problem.
It might just be a few muscles you haven't used for a long time but I'd spend a bit of time at the start of each session really thinking about your posture, and stop every so often to check you're still OK. It'll soon become a natural position, but until it does keep your mind on it.
I know it's tempting to just get on and play, but you could get into bad habits in the rush to make the most of your practice session by launching into playing without getting a few basics nailed.
Good luck, hope it's going well.
regards, Phil