Thanks Krysalyss, I'm not sure that it's sciatica though cause I'm pretty sure I've had that before and that was further around the back on my bum. Plus it felt more sharp, like nerve pain than this does. This feels like an ache and only happens during the night from about 2:30am, after I've been lying on my sides for a while. It also goes away as soon as I get up and walk around or stretch. But I will try the stretch you mentioned too!