So my friend has had this snake for a couple years now, I noticed recently he stopped paying it a whole bunch of attention and he ended up offering it to me since when i'm at his house I have it out and chill with it.

The problem is is that I hadn't seen the snake a awhile due to school and junk so when I actually got it home I noticed there was an issue. I googled and the only thing I could find similar was when a ball python has laid eggs.

Basically it is skinny, I can see its ribs and the first 1/3 of its body is like...concave? inside itself?

I looked up illnesses and common signs but it isn't showing any problems. It is alert and active like I remember and it isn't whistling or bubbling, not even gaping. I fed it a large rat and it took it fine, kept it down. It also shed after that meal, one piece if that matters. Its been about a week since that meal.

It isn't aggressive at all and even though I don't like handling it due to it seeming so fragile it has a great grip and seems to enjoy just climbing around my shoulders/head and chair.

Any thoughts at all? Is it just lack of feeding? Should I feed it bigger food or just more food?