Girls, hello! I'm 25 weeks pregnant, could you please tell me when to …

Girls, hello! I'm 25 weeks pregnant, could you please tell me when to eat, before or after going for a walk? There's such tension in my lower abdomen, like a stone, what is it? How critical is it? Only on ultrasound on Tuesday, as they said at the second screening that the placenta was low-lying, will they see if it has risen. Thanks in advance.