18 weeks constant cramp and pain when standing...

I have a constant cramp and pain ( goes from dull to sharp) when I'm standing or walking. It is located well, right where baby is. Low and above the pelvis right in the center. When I lay down it completly goes away. As soon as I'm up moving it comes back. I'm on my second dose of antibiotics for pneumonia, I took the 2 pills today and will take one for the next 4 days. I'm worried the medicine is affecting the baby? Or maybe I'm overthinking and it's just normal uterus stretching pain but it's really uncomfortable...anyone have any thoughts?