I've only been in business a cpl years, but it has happened to me before when the area being worked is already very tight and tender to touch. One of my once a week clients, due to overtime on her job at this time of year, has developed problems with her left bicep and I did more specific work on it than I'd been doing and started using some of James Waslaski (spelling) techniques. She said she was fine the next morning, but by noon was really sore, but the next day it was fine and she had better range of motion than she's had for a while. I try to always err on the side of caution though, as I have learned by trial and error to be careful being too aggressive with a new client. It just works better for me to let the client gain more trust and a better understanding of what to expect during and after a massage before I work too deeply.
In your case though, it turned out well and you have earned a new client.