I think the stiffness you describe is probably related indirectly to the effect of pregnancy hormones on your joints and ligaments. Although these hormones generally have a relaxing effect, the result can be some slight instability of the joints – especially of your pelvis and back. Combined with the increasing weight of your uterus, this may mean that when you are resting the joints ‘give’ and settle into an uncomfortable position – making it all the more harder for you to move when it is time to do so.
Your spine is under particular stress during pregnancy. As the uterus grows, many women find themselves leaning back slightly whilst walking or standing to compensate for the increased weight carried at the front of the body. This exaggerates the normal gentle curve of the spine, leading very often to backache as some muscles are over-stretched and others tightened. Oddly enough, this backache can be worse once you actually lie down and try to relax.
You may find it helps to do three things.
First, think carefully about your posture during the day. Try to use your abdominal muscles to ‘hug your baby closer’. Your bottom will automatically tuck in and the curve of your spine will flatten. Lift your ribcage, square your shoulders slightly, and ‘think tall’. Practise ‘pelvic tilts’ whilst standing against a wall: breathe in, then, as you breathe out, press your lower back into the wall. This will help straighten your back and relieve strain. You may also benefit from more specific exercises to help strengthen your tummy muscles; look out for antenatal exercise classes in your area, and relevant leaflets and videos.
Secondly, think about ways of getting more comfortable at night. Try sleeping well over on your side (this is better for the circulation to your placenta, and may help encourage your baby into a good position for birth). Put a pillow in the small of your back, to support this area should you roll over during the night. Put another small pillow under your bump, to reduce pressure on the abdominal muscles. Finally, bend your uppermost leg forwards and support it with another pillow. This helps keep your legs slightly apart, thus reducing pressure on the hip joints.
Thirdly, make sure that you are drinking plenty of water (in addition to that in tea and coffee). Even a small degree of dehydration can lead to joint stiffness, whether or not one is pregnant.
I don’t think the stiffness is directly related to the position of your baby. It is normal to feel some abdominal discomfort if your baby is lying so that there is pressure on your bladder, or certain nerves. Lying well on your side, so your uterus hangs forward slightly may help relieve this kind of discomfort. I hope you get some relief soon.