Swelling below vent, should I be worried??

[lapsarian]

I noticed yesterday that my hen Nesty (black australorps) is swollen below her vent, about the size of a lemon. The skin is reddish and it is warm to the touch. It doesn't seem to be tender; she lets me feel it without complaint. she seems to be eating, drinking and acting normally, and I think she may have laid an egg today but I'm not totally sure.

what could this be and what should I do?
thanks!