Aiyah, the weight gain and discomfort in third trimester is so real lah, a few of my mum friends complained about the same back pain and sore feet, especially when they couldn't find a comfortable sleeping position! Try using a pregnancy pillow between your knees and under the bump, it really helps ease the lower back strain, and maybe soak your feet in warm water before bed to help with the soreness. Best to mention all these aches to your doctor at your next checkup lah, especially if the pain gets worse, so they can give you proper advice and maybe refer you to a physio if needed.