The whole "Penis too big thing" always makes me chuckle!
"Hey lass thanks for the compliment but even with the girth and length of my humongous Dong it can't be as big as a Babies head and shoulders passing out of your Vag!"
Time of month more than likely, an old hoor trick to not lose business, frankly I think it is poor form and should not be encouraged!
Either Prossies and their Madams/Sergei should be honest when a punter calls and say due to time of month limited services only or it is their monthly holiday from work! To be fair some do advertise as such when it is the time of the month and if she has a reputation for a good blow job and it is a quickie then why not.
That or you have a Penis the size of an elephants, whereupon well done mate!