According to PoultryDVM (not necessarily the most scientifically rigorous source) the dose of aspirin for chickens is 5mg/kg given three times a day. So an individual dose of 15mg is probably right giving a total daily does of 45mg.
So yes, she got too much. Not quite double the correct (per PoultryDVM) dose.
However, she will likely be fine.
I found two relevant studies.
- One gave 10mg/kg every day for many days and monitored blood for adverse signs. They started seeing adverse (not fatal) signs on day 3. You only gave it once.
- Another study was looking at aspirin as a feed additive. It is a badly written paper which claims they gave 100mg/kg of aspirin (along with vitamin C) but it may be that they gave 100mg/chicken. Either way it is more than you gave her and their results concluded that all that aspirin boosted growth (it was a broiler study).
So, I think she will be fine. But if you are fretting you could try giving her some activated charcoal which is the treatment for aspirin overdose.
I am glad you prompted me to look it up because in my head I assumed a whole baby aspirin was the right dose!