It can also be a result of
good underwater posture, but should fade over time if you dive regularly.
Proper horizontal trim should leave you in a sort of parachutists' freefall postion - with a slight arch to your back. If your muscles aren't used to this, it can lead to initial aching. As your muscles develop and get used to the position, that ache should cease.
Be aware of that 'freefall' position.... hips and shoulders held high, tension in the stomach to support the mid-section (people normally ache because of weak stomach muscles) and legs bent upwards at 90 degrees behind..
Have a look at the video
HERE for a demo of great trim position and bouyancy.
The key to obtaining great trim is to ensure that you are correctly weighted and that your weighting and tank are properly located to ensure an even distribution over your center of gravity. Ankle weights are not necessary. Backplate systems assist with this.
Illustration showing centre of gravity and even distribution weight.
Nice flat horizontal trim - showing straight line through knees, hips, spine and shoulders.
Below - divers practising good trim (can make back ache initially)
Not so good trim...
Less efficient trim. Sloping of the body, which rises from knees to hips to shoulders. Often caused by overweighting and bad bouyancy, leading to the need to kick downwards to maintain level. Causes extra strain. Diver works hard, using more energy and depleting air quicker.