Signs of labor coming on or dehydration?

An

Sometimes I wake up in the middle of the night with what it feels like back contractions that also wrap around to my lower abdomen. This is my first but I'm assuming this is what back contractions feel like because it hurts and wraps around. My stomach doesn't tighten though. They are not BH because I get those quite often so I know the difference. I know I'm not in active labor because they don't come back consistently. But when they come it only happens at night they hurt so bad it wakes me up. Do you think I'm dehydrated or maybe just my body getting ready? I'll be 35 weeks tomorrow.