Girls, dear, can you tell me, it's week 18 of pregnancy. I feel good o…

Girls, dear, can you tell me, it's week 18 of pregnancy. I feel good overall, nothing hurts, there's just a slight constipation. But when I lie down on my back, I feel like the belly below the navel tightens, I feel a ball, the belly is firm, and the sensations aren't pleasant. Do you think this is the onset of toneus? What's dangerous about it, and what can I do? I can see the doctor only on Sunday.