Ahhh, I remember this phase! At 19 weeks your hips are starting to shift and loosen up for labour, so sleeping on your side puts pressure on those ligaments, totally normal tapi memang uncomfortable la. A few of us tried putting a pillow between our knees and under the bump, and that really helped ease the side aching tengah tidur. If it gets worse or you start feeling sharp pain instead of just that dull ache, better check with your doctor just to be safe, but honestly sounds like one of those pregnancy things that comes and goes. Hang in there, second trimester can be a bit achey but it does get better once you find your sleeping position!