Ache while sleeping

The past few nights I've woken in the middle of the night with my lower abdomen aching. It isn't traditional cramping in that it doesn't come and go but rather feels like a the muscle has cramped some and won't let go. Sometimes it goes away after a few minutes, sometimes I fall back asleep and it is gone when I wake, sometimes it lasts a few hours into my morning. No bleeding and it isn't gas. I'm 19 weeks. Does this seem normal? Is it possible the weight of my bump is pulling down when I sleep on my side? I have an appointment in 5 days and will ask then if nothing changes.