You stated that the pain came on after a vigorous ride, went away in a day, and then came back after a another vigorous ride. If the pain is mainly a burning type pain, then I would lead toward tendonitis. The pain from stress fractures often moves around some, and it also does not come on all of a sudden. To get a stress fracture, you have to ignore or try and "fight through" the pain for a decent period of time. Tendonitis, while an over-use injury (stress fractures are in this class as well), can come on rather suddenly, especially when you have a quick and dramatic increase in training intensity or duration.
I suppose bike fit could be part of the problem, but if you have logged 2800 miles pain free, and more importantly, nothing has changed recently (new bike, new pedals, new cycling shoes, different saddle, etc), I wouldn't bet on it.
Just my "not intended to be construed as medical advice in any way" $0.02 worth.
D