To me, this looks like a regional or family variation in meaning.
To me (born & raised in the American South - aka the Confederate States of America back in 1860), what the OP said would have been interpreted as "I won't be going unless I call you back to tell you that I will be going".
In other regions or other families - such as the OP's friend - it apparently means "I will probably be going but will call you back later".
This is why a clear "I need to check my calendar - I will be getting my own ticket if I find that I am free to go" would probably have been better understood. But going out and buying a ticket without a clear request and offer to pay for the ticket is going out on a limb..............what if the OP had gotten home that evening to find a "surprise party" being held in her honor instead of being sick? Would she be held at fault then?
The problem seems to be that some people want to read into what happened a little more than it is possible to glean from the post *as written*. We weren't there to ask where the OP and her friend were raised, the custom in the area where they live now, their body language and tone of voice during the conversation, and whether or not the OP's friend has a history of thinking "the more the merrier" when putting together a theater party (as in the number of people going to the theater in one GROUP). Or even if the OP had gone to a similar function with the friend earlier and the friend wanted to repeat that experience...........because she enjoyed the OP's company.
Perhaps this is best taken as a caution to be CLEARER in communicating with everyone in the future.......as the number of mind-readers in the world seems to be very small.