it's really quite common, over the years i've noticed a fair number of women mention this post-partum, and have had it resolve as sleep gets better and health stabilises for mum.
However, and now don't panic yourself but in my case, well, i've developed rheumatoid arthritis , and one of the symptoms any of us with it recognise is morning stiffness.
In a case like mine it's because pregnancy is a major immune event, and an immune system, depressed during pregnancy to stop you rejecting the baby, can bounce back too hard after the birth and give rise to autoimmune disease, where it thinks bits of you are the enemy...in my case my joint linings, gut and thyroid.
with ds1 i had auto-immune hypothyroiditis develop (which can also cause joint pain!) and with ds2, RA and coeliac disease.
though hips is an uncommon site for RA, so don't you worry needlessly. also, some fluctuation in thyroid function is common post-partum and will settle down by itself most frequently.
ask gp for thryoid and RA tests screen if you are worried. more about RA here And please don't worry too much, it's more likely not RA and if it is RA, well the drugs work OK.