Good day. Ladies, could you help me out? It's probably an overused top…

Good day. Ladies, could you help me out? It's probably an overused topic, but I'm still nervous. My stomach hardens when I strain my abdominal muscles. When I get up from bed or even a chair. When I lie down for 5 minutes, it softens again, but as soon as I stand up, it hardens again and so all day long. I saw a doctor. An ultrasound didn't show any tonus issues, then on the chair, my stomach stood up again. The belly doesn't hurt or pull. They said everything is in order at the neck. They prescribed a bed rest regime, magnesium and candles for spasms. Is this seriously dangerous? The baby is very active