Is it normal for my bump to be getting really firm?

I'm 16 weeks pregnant, and over the last week my bump has become quite firm and I can no longer lie on it, even on the side of it. I know my tummy will become harder but is it normal at 4 months? I thought it would happen later. It feels really tight and I suddenly have a lot more gas (sorry TMI) whereas before I could not p any wind. Is it just my organs shifting around a bit that is making the gas? Sorry again but it's all so weird and my body is just freaking me out, I'm not used to the changes.