Wearing a hip support belt during the day will keep your hips & spine in place whilst your body returns to it’s normal state. Use a Mumanu Pregnancy Pillow at night to keep your spine in proper alignment. If your mattress is too soft, then put some hard planks/board under the mattress to firm it up a bit.
Homeopathic remedy for back and pelvic pain after pregnancy is Bellis Prurenis 6/12/30C.