I would like to point out that a L (200 g) to XL (400 g) rat is a full grown, adult female nursing. I highly doubt that 600g snake can take an XL(400 g), though I don't doubt it'd try. And yes, that snake does seem to look like 600-800 grams.
I'm guessing your probably feeding her a 5- 6 week old rat or medium. Topping out at maybe 150 grams, if that.
Even then, you haven't probably haven't kept up with feeding her every week like you've said. You said you missed a week, what's really keeping us from believing it was just a week and not more?
There are many people that come to this site, feeding less than normal, some people have been told by pet stores to feed once a month in some cases. It's not going to hurt your image at this point to just say you haven't been feeding her as much as you should, because you didn't know better.
Either you haven't been feeding her as much as you claim, or she is literally being eaten alive by internal parasites.
When you feed her, how big is the lump after feeding? Please, be very descriptive and precise.
When is your VET appointment for? I believe the Vet can get a fecal straight from the snake. (?) You should get her in ASAP.
