I second renenet. Is there no possibility she is gravid? Pics are necessary. Was she purchased only recently? She may have been sold to you gravid if this is the case.
I am not sure whether knob tails have the ability to retain sperm (if someone could let me know) as if possible this, may be a possibility???
If you post pics and no one can identify FOR SURE what the issue at hand is, I recommend you see a vet. May only be that she's gravid but you'd rather be safe and pay a bit than sorry.
Best of luck!