First, I'd most definitely stop. I'm not into inflicting pain. I'd see if we could find a mutually agreeable solution.

Yes, it could be an excuse that's spreading. It could be that a prior client was rough, not stopping, causing swelling.

What was their attitudes towards YOU at the beginning? Eager for business, or, "Let's get this over with"? WADR, what was your attitude? "Let's have some fun", or "Bend over and brace"? Perhaps they were just tiring that you were taking longer...

Obviously, again WADR, they're not interested in repeat business with you. Have to ask yourself why? (rhetorical question). Most want a client list - money in the bank.

You might ask at the beginning if she's OK with "you and yours"? You might consider bringing a bottle of QUALITY lubrication and offering BEFORE things get tight, using SLOW and steady until things loosen up. And use lubricated condoms (nice that they can also include spermicide).

Bottom line: I'd take it as a "the money is not worth it for whatever reason" and "not interested in future business" and act accordingly, one-n-done. Regardless, now you KNOW what her experience is like with YOU.